Method of manufacturing a vapor cell

ABSTRACT

In a general aspect, a vapor cell includes a body defined by a stack of layers bonded to each other. The stack of layers includes a first end layer disposed at a first end of the body and a second end layer disposed at a second, opposite end of the body. Intermediate layers extend between the first and second end layers and define an internal cavity extending through the body between the first end layer and the second end layer. The stack of layers also includes first and second sets of tabs. The first set of tabs extends outward from the intermediate layers on a first exterior side of the body, and the second set of tabs extends outward from the intermediate layers on a second exterior side of the body. The vapor cell also includes a vapor or a source of the vapor disposed in the internal cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application. Ser. No.17/480,443, filed Sep. 21, 2021 and entitled “Interlockable VaporCells”, which claims priority to U.S. Prov. App. No. 63/174,289, whichwas filed on Apr. 13, 2021 and entitled “Interlockable Vapor Cells.” Thedisclosures of the priority applications are hereby incorporated byreference in their entirety.

BACKGROUND

The following description relates to vapor cells, which in someconfigurations, can be interlockable.

Vapor cells are manufactured by sealing a vapor or gas within anenclosed volume. The vapor or gas can be used as a medium to interactwith electromagnetic radiation generated by an external source. Lasersmay be directed through the vapor cell, and a response to the lasers canbe used to determine properties of the electromagnetic radiation andserve as sensor of electromagnetic radiation.

DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram, in perspective view, of an example vaporcell 100;

FIG. 1B is a schematic diagram of the example vapor cell of FIG. 1A froma bottom perspective;

FIG. 1C is a schematic diagram, in perspective view, of the examplevapor cell of FIG. 1A, but in which the example vapor cell has aspherical shape and an internal cavity with a spherical shape;

FIG. 1D is a schematic diagram, in perspective view, of a half portionof the example vapor cell of FIG. 1C;

FIG. 1E is a schematic diagram of the example vapor cell of FIG. 1A, butin which the example vapor cell has an octagonal shape and lacks amechanical interface;

FIG. 1F is a schematic diagram of the example vapor cell of FIG. 1A, butin which the example vapor cell 100 has a square shape and lacks amechanical interface;

FIG. 1G is a schematic diagram, in perspective view, of three instancesof the example vapor cell of FIG. 1A in which two instances are coupledto each other;

FIG. 2A is a schematic diagram, in perspective view, of five instancesof the example vapor cell of FIG. 1A that are interlocked with eachother to form a “tee” subassembly;

FIG. 2B is a schematic diagram, in perspective view, of eight instancesof the example vapor cell of FIG. 1A that are interlocked with eachother to form an octagon subassembly;

FIG. 3A is a schematic diagram, in perspective view, of five instancesof the “tee” subassembly of FIG. 2A that are interlocked with each otherto form an arcuate assembly;

FIG. 3B is a schematic diagram, in top view, of four instances of theoctagon subassembly of FIG. 2B that are interlocked with each other toform a tiled pattern;

FIG. 4A is an electromagnetic simulation, shown in graph form, of anexample configuration of a vapor cell, in which a plurality of holes ispresent in each intermediate layer;

FIG. 4B is an electromagnetic simulation, shown in graph form, of anexample configuration of the vapor cell of FIG. 4A, but in which theplurality of holes is not present; and

FIG. 5 is an electromagnetic simulation, shown in graph form, ofelectric field distributions inside of various example configurations ofvapor cells having perforated walls;

FIG. 6A is a schematic diagram of an example vapor cell having an arrayof cavities therein; and

FIG. 6B is a schematic diagram of the example vapor cell of FIG. 6A, butin which intermediate layers of the example vapor cell lack tabsextending outward therefrom.

DETAILED DESCRIPTION

In a general aspect, a vapor cell may include a body defined by a stackof layers bonded to each other, such as a laminated or bonded stack oflayers. This configuration can allow the vapor cell to be manufacturedin large numbers and with tight tolerances. The configuration also canallow structuring in three dimensions and tiling multiple instances ofthe vapor cell for building arrays. During operation, the vapor cellsmay have a more uniform RF electric field in the center of theirinterior, thereby making measurements more precise and accurate. In someexamples, the average RF electric field amplitude in the interactionregion of the vapor cell can be within 1% of the incident RF amplitudewith less than 1% variation across the intended interaction region. Thebackground gases in the vapor cell can also be lower than the vaporpressure (e.g., an alkali vapor pressure), so that collisionalbroadening is reduced or minimized. In some instances, the vapor cellhas a lower—and in some configurations significantly lower—radarscattering cross-section than its geometric cross-section. Atransparency criterion can be met using a vapor cell geometry that canbe scaled for frequencies beyond 40 GHz. The vapor cell interactionregion can also be scaled, taking frequency into account, to reduce orminimize transit time broadening. In some cases, the vapor cell can befiber coupled and configured to work in counter-propagating,co-propagating, or both, geometries of light propagation. In someconfigurations, multiple instances of the vapor cell may be interlockedwith each other through a mechanical interface. Non-interlockableconfigurations, however, are possible.

FIGS. 1A-6B present schematic diagrams of example configurations for avapor cell along with example arrangements of some possible tilingpatterns or assemblies. The example vapor cells shown in FIGS. 1A-6Binclude a laminated body of layers, such as that constructed from glassand high resistance (float) silicon. The schematic diagrams illustratealternating layers of dielectric material (e.g., alternating layers ofsilicon and glass) but other arrangements of dielectric material arepossible. The laminated configuration makes it possible to structure thevapor cell in three dimensions and eases the manufacturing requirementsbecause individual layers can be cut in two dimensions from large, highquality wafers or substrates. In some variations, the layers areanodically bonded together to form the laminated configuration. The topand bottom windows (or end layers) can be made from anoptically-transparent dielectric material, such as, for example, aborosilicate glass or an aluminosilicate glass. Each unit can be fibercoupled and the windows can have optical mirrors and filters depositedon them. The configuration of the vapor cells is flexible and allows forthree-dimensional structuring and efficient manufacturing. In somevariations, the vapor cells have a mechanical interface (e.g.,interlocking tabs) that can be used for tiling multiple vapor cellstogether.

Now referring to FIG. 1A, a schematic diagram is presented, inperspective view, of an example vapor cell 100. FIG. 1B presents aschematic diagram of the example vapor cell 100 of FIG. 1A from a bottomperspective. The example vapor cell 100 includes a body 102 defined by astack of layers 104 bonded to each other (e.g., a laminated body definedby the stack of layers 104). The stack of layers 104 may include one ormore external surfaces defining an outer shape of the body 102 (e.g., anouter cuboidal shape, an outer spherical shape, an outer rectangularshape, an outer ellipsoidal shape, etc.). For example, each layer of thestack 104 may include an outer perimeter surface that defines across-section of the outer shape at a location of the layer (e.g., asquare cross-section, a circular cross-section, a rectangularcross-section, etc.). The cross-section may remain constant along thestack of layers, vary along the stack of layers 104, or some combinationthereof. To vary the cross-section, at least two adjacent layers mayhave respective outer perimeter surfaces that differ, relative to eachother, in one or both of shape and size. FIGS. 1A and 1B depict theexample vapor cell 100 as having an outer cuboidal shape with across-section that is constant along the stack of layers. However, othershapes are possible (e.g., see FIGS. 1C and 1E).

In many implementations, the stack of layers 104 are formed ofdielectric material that is transparent to a target electromagneticradiation measured by the example vapor cell 100. The dielectricmaterial may allow the stack of layers 104 to absorb no more than 1% ofthe target electromagnetic radiation when such radiation passes throughthe body 102. The dielectric material may also be an insulating materialhaving a high resistivity, e.g., ρ>10⁸ Ω·cm, and may correspond to asingle crystal dielectric material, a polycrystalline dielectricmaterial, or an amorphous dielectric material.

In some implementations, a “transparent” dielectric material is one thatminimizes the absorption and scattering of the target electromagneticradiation by the body 102. In these implementations, the stack of layers104 may transmit the target electromagnetic radiation withoutsignificant distortion to the back of the body 102 or its interior. Insome variations, the stack of layers 104 has a radar scatteringcross-section less than its geometric cross-section. The stack of layers104 may also have absorption of less than 1% for the targetelectromagnetic radiation. For example, the stack of layers 104 may beformed, in whole or in part, of float-type silicon. The loss tangent offloat-type silicon may be less than 10-3 for frequencies up to 400 GHz.In certain applications, such as antenna testing, high transparency isrequired to minimize reflections in a test environment and to makeaccurate measurements of power over more than one spatial plane orsurface. In some cases, the dielectric material forming the stack oflayers 104 allows a target electromagnetic field inside the examplevapor cell 100 to be within 1% of the target electromagnetic field whenincident thereon.

For example, one or more layers of the stack 104 may be formed ofsilicon, such as a single-crystal silicon. In another example, one ormore layers of the stack 104 may be formed of an amorphous material thatincludes silicon oxide (e.g., SiO₂, SiO_(x), etc.), such as vitreoussilica, a borosilicate glass, or an aluminosilicate glass. In yetanother example, one or more layers of the stack 104 may be formed of anoxide material such as magnesium oxide (e.g., MgO), aluminum oxide(e.g., Al₂O₃), silicon dioxide (e.g., SiO₂), titanium dioxide (e.g.,TiO₂), zirconium dioxide (e.g., ZrO₂), yttrium oxide (e.g., Y₂O₃),lanthanum oxide (e.g., La₂O₃), and so forth. The oxide material may benon-stoichiometric (e.g., SiO_(x)), and may also be a combination of oneor more binary oxides (e.g., Y:ZrO₂, LaAlO₃, BaLa₂Ti₄O₁₂, etc.). Theoxide material may also be a single crystal material, a polycrystallinematerial, or an amorphous material. In still yet another example, one ormore layers of the stack 104 may be formed a non-oxide material such assilicon (Si), diamond (C), gallium nitride (GaN), calcium fluoride(CaF), and so forth. Other dielectric materials are possible.

The stack of layers 104 includes a first end layer 106 disposed at afirst end 108 of the body 102. The first end layer 106 is formed of afirst type of dielectric material that is optically transparent. Thefirst type of dielectric material may be optically transparent toinfrared wavelengths of electromagnetic radiation (e.g., 700-1000 nm),visible wavelengths of electromagnetic radiation (e.g., 400-7000 nm),ultraviolet wavelengths of electromagnetic radiation (e.g., 10-400 nm),or some combination thereof. The stack of layers 104 also includes asecond end layer 110 disposed at a second, opposite end 112 of the body102. The second end layer 110 is formed of dielectric material, forexample, as described above in relation to the stack of layers 104. Thedielectric material may be different than first type of dielectricmaterial forming the first end layer 106. However, in some variations,the second end layer 110 is formed of the first type of dielectricmaterial.

In some variations, the first type of dielectric material includessilicon oxide (e.g., SiO₂, SiO_(x), etc.), such as found within quartz,vitreous silica, or a borosilicate glass. In some variations, the firsttype of dielectric material includes aluminum oxide (e.g., Al₂O₃,Al_(x)O_(y), etc.), such as found in sapphire or an aluminosilicateglass. In some variations, the first type of dielectric material is anoxide material such as magnesium oxide (e.g., MgO), aluminum oxide(e.g., Al₂O₃), silicon dioxide (e.g., SiO₂), titanium dioxide (e.g.,TiO₂), zirconium dioxide, (e.g., ZrO₂), yttrium oxide (e.g., Y₂O₃),lanthanum oxide (e.g., La₂O₃), and so forth. The oxide material may benon-stoichiometric (e.g., SiO_(x)), and may also be a combination of oneor more binary oxides (e.g., Y:ZrO₂, LaAlO₃, BaLa₂Ti₄O₁₂, etc.). Theoxide material may also be a single crystal oxide material, apolycrystalline oxide material, or an amorphous oxide material. In othervariations, the first type of dielectric material is a non-oxidematerial such as diamond (C), calcium fluoride (CaF), and so forth.Other dielectric materials are possible.

The stack of layers 104 additionally includes intermediate layers 114between the first and second end layers 106, 110. The intermediatelayers 114 may be formed of dielectric material, for example, such asdescribed above in relation to the stack of layers 104. The intermediatelayers 114 define an internal cavity 116 that extends through the body102 between the first end layer 106 and the second end layer 110. Theinternal cavity 116 may run along an axis that is perpendicular to thestack of layers 104, although other orientations are possible (e.g., acanted axis). Furthermore, although FIGS. 1A and 1B depict the internalcavity 116 as having a cylindrical shape, other shapes are possible(e.g., rectangular, hexagonal, ellipsoidal, spherical, etc.). Forexample, FIG. 1C presents a schematic diagram, in perspective view, ofthe example vapor cell 100, but in which the example vapor cell 100 hasa spherical shape and an internal cavity 116 with a spherical shape.FIG. 1D presents a schematic diagram, in perspective view, of a halfportion of the example vapor cell 100 of FIG. 1C.

In some implementations, each intermediate layer 114 includes athrough-hole that defines a portion of the internal cavity 116 throughthe intermediate layer 114. In these implementations, the through-holesmay be selectively configured such that the intermediate layers 114,when stacked, define a target three-dimensional volume for the internalcavity 116 (e.g., a sphere, a frustrum, an inclined parallelepiped,etc.). The through-holes may be configured in any combination of shape,size, and location. Other characteristics are possible. In somevariations, each through-hole is identical in shape and size. In thesevariations, the internal cavity 116 may have a cross-section that isconstant through the intermediate layers 114. FIGS. 1A and 1B show anexample where the through-holes are circular, share a common size, andare aligned along a direction perpendicular to the stack of layers 104.As such, the through-holes define a cylindrical volume for the internalcavity 116. In some variations, at least two adjacent intermediatelayers 114 have respective through-holes that differ, relative to eachother, in one or both of shape and size. In these variations, theinternal cavity 116 may have a cross-section that varies, at least inpart, through the intermediate layers 114. FIGS. 1C and 1D show avariation where the through-holes are circular, but differ in size. Thethrough-holes are aligned along a direction perpendicular to the stackof layers 104 to define a spherical volume for the internal cavity 116.

Other features of the intermediate layers 114 may be used to define thetarget three-dimensional volume. For example, two or more intermediatelayers 114 (e.g., adjacent intermediate layers) have differentthicknesses. As another example, the through-hole of an intermediatelayer may be defined by an internal perimeter of the intermediate layerthat includes an internal perimeter surface. The internal perimetersurface may be angled, beveled, or rounded to assist in defining thetarget three-dimensional volume.

The target three-dimensional volume may be selected to shape a profileof electromagnetic radiation that forms in the internal cavity 116 whenthe electromagnetic radiation is incident on the example vapor cell 100.For example, the target three-dimensional volume may be selected toconcentrate the incident electromagnetic radiation in a center of theinternal cavity 116. Such shaping can increase an amplitude of theincident electromagnetic radiation in the internal cavity 116 (e.g., anamplitude of the electric field or magnetic field), making the examplevapor cell 100 more sensitive to the incident electromagnetic radiation.The target three-dimensional volume may also be selected to make theprofile of the incident electromagnetic radiation more uniform, such asacross a desired region within the internal cavity 116. Increasing theuniformity can increase an amount of vapor that interacts with theincident electromagnetic radiation, thereby increasing a sensing regionwithin the internal cavity 116. The target three-dimensional volume mayadditionally be selected to reduce a thickness of walls surrounding theinternal cavity 116, thereby allowing the body 102 of the example vaporcell 100 to be more transparent to the incident electromagneticradiation. Other benefits are possible.

The target three-dimensional volume may also be selected together withan outer shape of the body 102. Certain volume and outer shapecombinations may improve the performance of the example vapor cell 100for a target application. For example, the internal cavity 116 may havea spherical volume and the body 102 may have an outer spherical shape(e.g., as shown in FIGS. 1C-1D). This combination may allow for easiermodeling of electromagnetic field profiles inside and outside of theexample vapor cell 100. The ease of such modeling may be used to improvethe performance of the example vapor cell 100 in metrological or otherapplications.

In some implementations, such as those that mitigate or prevent heliumpermeation, the stack of layers 104 includes layers formed ofaluminosilicate glass or borosilicate glass (e.g., Pyrex @). The stackof layers 104 may be coated with materials that exhibit a low heliumpermeation. Various optical coatings (e.g., Bragg mirrors) can also bedeposited on the first and second end layers 106, 110 to act as highreflectors and filters for optical signals used for laser preparationand signals. Waveplates can be used to control the polarization alongwith polarization-preserving fiber. Moreover, the example vapor cell 100can be fiber optically coupled using GRIN (gradient index) lensescentered on an end layer of the vapor cell (e.g., the first end layer106 or the second end layer 110).

The example vapor cell 100 also includes a vapor (not shown) disposed inthe internal cavity 116 defined by the intermediate layers 114. Thevapor may include constituents such as a gas of alkali-metal atoms, anoble gas, a gas of diatomic halogen molecules, a gas of organicmolecules, or some combination thereof. For example, the vapor mayinclude a gas of alkali-metal atoms (e.g., K, Rb, Cs, etc.), a noble gas(e.g., He, Ne, Ar, Kr, etc.), or both. In another example, the vapor mayinclude a gas of diatomic halogen molecules (e.g., F₂, Cl₂, Br₂, etc.),a noble gas, or both. In yet another example, the vapor may include agas of organic molecules (e.g., acetylene), a noble gas, or both. Othercombinations for the vapor are possible, including other constituents.

Additionally or alternatively, the example vapor cell 100 may include asource of the vapor disposed in the internal cavity 116 defined by theintermediate layers 114. The source of the vapor may generate the vaporin response to an energetic stimulus, such as heat, irradiation, and soforth. For example, the vapor may correspond to a gas of alkali-metalatoms and the source of the vapor may correspond to an alkali-metal masssufficiently cooled to be in a solid or liquid phase when disposed intothe cavity 116. In some variations, the source of the vapor resides inthe internal cavity 116, and the source of the vapor includes a liquidor solid source of the alkali-metal atoms configured to generate a gasof the alkali-metal atoms when heated. In some variations, the source ofthe vapor may function, in part, as a getter.

In some implementations, all of the intermediate layers 114 are formedof the same type of dielectric material (e.g., the first type ofdielectric material discussed above). In some implementations, the vaporcell can include multiple different types of intermediate layers 114(e.g., layers of different materials, different thicknesses, differentstructural features, etc.). In some cases, the intermediate layers 114may include two or more subsets of layers defining an arrangement ofintermediate layers. In these cases, the layers in each subset may sharea characteristic in common. For example, the intermediate layers 114 mayinclude a first subset of layers 114 a and a second subset of layers 114b, such as shown in FIGS. 1A and 1B. The first subset of layers 114 amay be formed of the first type of dielectric material (e.g., one thatincludes silicon oxide), and the second subset of layers 114 b may beformed of a second, different type of dielectric material (e.g.,silicon). In another example, the intermediate layers 114 may include afirst subset of layers having a thickness different than a second subsetof layers. Other common-shared characteristics are possible for thesubsets of layers (e.g., a through-hole shape, a through-hole size, thepresence of additional cavities, tabs for a mechanical interface, etc.).

In some implementations, such as shown in FIGS. 1A and 1B, the secondend layer 110 is formed of the first type of dielectric material and thelayers 104 are ordered in the stack to alternate between the first typeof dielectric material and the second type of dielectric material. Forexample, the first and second end layers 106, 110 may be formed of thefirst type of dielectric material and the first and second subset oflayers 114 a, 114 b may alternate in sequence within the intermediatelayers 114. In this configuration, the first subset of layers 114 a maybe formed of the first type of dielectric material and the second subsetof layers 114 b may be formed of the second type of dielectric material.Moreover, the first subset of layers 114 a may include the outerintermediate layers (e.g., the first and last intermediate layers in thesequence). Such an alternating configuration may allow easier bonding ofthe stack of layers 104 to each other. The alternating configuration mayalso reduce an effective dielectric constant of the body 102. Otherbenefits are possible.

In some implementations, the stack of layers 104 includes sets of tabs118 extending outward from one or more exterior sides 120 of the body102. Each set of tabs 118 may define a mechanical interface that allowsthe example vapor cell 100 to couple with (e.g., interlock with) anothervapor cell. In some variations, the sets of tabs 118 include one or moretabs that are an integral part of a layer. For example, an intermediatelayer 114 may have inner perimeter surface disposed within an outerperimeter surface. The inner perimeter surface may define thethrough-hole of the intermediate layer 114 and the outer perimetersurface may define a tab extending outward from an exterior side of theintermediate layer 114. In another example, the first end layer 106 mayhave an outer perimeter surface that defines a tab extending outwardfrom an exterior side of the first end layer 106.

In some implementations, the example vapor cell 100 lacks a mechanicalinterface (e.g., lacks tabs). FIG. 1E presents a schematic diagram ofthe example vapor cell 100 of FIG. 1A, but in which the example vaporcell 100 has an octagonal shape and lacks a mechanical interface.Similarly, FIG. 1F presents a schematic diagram of the example vaporcell 100 of FIG. 1A, but in which the example vapor cell 100 has asquare shape and lacks a mechanical interface. The example vapor cells100 of FIGS. 1E and 1F include a plurality of holes 128 between theinternal cavity 116 and an exterior surface of the body 102. Theplurality of holes 128 may be selected in shape, size, and number toconfigure the example vapor cells 100 to detect a target radiation (orimprove such detection). The plurality of holes 128 may also reduce aradar cross-section of the example vapor cells 100. Features of theplurality of holes 128 are described further in relation to FIGS. 4A-5.

FIGS. 1A and 1B illustrate the stack of layers 104 as having a set oftabs 118 extending outward from each exterior side 120 of the body 102.Moreover, the sets of tabs 118 are illustrated as defining a singular,centrally positioned column on each exterior side 120. However, otherconfigurations are possible. For example, the stack of layers 104 couldinclude a set of tabs 118 extending outward from only a single exteriorside 120 of the body 102. As another example, the stack of layers 104could have a set of tabs 118 defined by a single tab that extendsoutward from a single exterior side 120. The single tab could bepositioned non-centrally on the single exterior side 120. In yet anotherexample, the stack of layers 104 could include a set of tabs 118 alignedwith each other along a first dimension on an exterior side 120.Adjacent pairs of tabs along the first dimension may be separated by agap. In some cases, the gap may correspond to the thickness of a singleintermediate layer 114 (e.g., the tabs 118 extend outward from everyother intermediate layer 114). However, the gap may correspond to otherthicknesses (e.g., two intermediate layers 114). Moreover, the gap neednot be the same for each adjacent pair of tabs.

In implementations where the intermediate layers 114 include the firstsubset of layers 114 a and the second subset of layers 114 b, the stackof layers 104 may include a first set of tabs 118 a and a second set oftabs 118 b. The first set of tabs 118 a extends outward from the firstsubset of layers 114 a on a first exterior side 120 a of the body 102.The second set of tabs 118 b extends outward from the second subset oflayers 114 b on a second exterior side 120 b of the body 102. In someimplementations, the first subset of tabs 118 a extends from a center ofthe first exterior side 120 a of the body 102, and the second subset oftabs 118 b extends from a center of the second exterior side 120 b ofthe body 102. Here, the tabs within each subset 118 a, 118 b are alignedwith each other on their respective exterior sides along a firstdimension (e.g., from the first end layer 106 to the second end layer110), and they are each centered on the respective exterior side along asecond dimension that is perpendicular to the first dimension. In someimplementations, each tab in the first set 118 a is an integral part ofa respective layer in the first subset of layers 114 a, and each tab inthe second set 118 b is an integral part of a respective layer in thesecond subset of layers 114 b.

In some implementations, such as shown in FIGS. 1A and 1B, the body 102has four exterior sides 120 defining a square cross-section of the body102. The four exterior sides 120 include the first and second exteriorsides 120 a, 120 b. The first and second exterior sides 120 a, 120 b maybe adjacent to each other, or alternatively, opposite to each other. Insome variations, the four exterior sides 120 include the third andfourth exterior sides 120 c, 120 d. In these variations, the stack oflayers 104 includes a third set of tabs 118 c extending outward from thefirst subset of layers 114 a on the third exterior side 120 c of thebody 102. The body 102 also includes a fourth set of tabs 118 dextending outward from the second subset of layers 114 b on the fourthexterior side 120 d of the body 102. The first, second, third, andfourth sets of tabs 118 a, 118 b, 118 c, 118 d may be aligned withrespective centers of the first, second, third, and fourth exteriorsides 120 a, 120 b, 120 c, 120 d.

During operation of the example vapor cell 100, one or more opticalsignals (e.g., laser light) may interact with the vapor in the internalcavity 116. For example, an optical signal may enter the internal cavity116 through the first end layer 106 and then exit the internal cavity116 through the second end layer 106. In another example, an opticalsignal may enter the internal cavity 116 through the first end layer106, reflect off an interior surface of the second end layer 106, andthen exit the internal cavity 116 through the second end layer 106. Iftwo or more optical signals are used, copropagating orcounter-propagating modes of operation may be established. In theco-propagating mode, each optical signal traverses the internal cavity116 along the same direction. In the counter-propagating mode, eachoptical signal traverses the internal cavity 116 along opposingdirections. Examples of propagation modes for vapor cells, includingexamples of operating vapor cells, are described in U.S. Pat. No.10,509,065 entitled “Imaging of Electromagnetic Fields.”

To assist in propagating optical signals through the internal cavity116, the first and second end layers 106, 110 may include one or moreoptical coatings. Examples of such optical coatings include a reflectivecoating, an anti-reflective coating, a filter coating, a polarizingcoating, and so forth. In some implementations, the first end layer 106includes an interior surface covering a first opening 122 of theinternal cavity 116 adjacent the first end layer 106. The first endlayer 106 also includes an exterior surface opposite the interiorsurface. In these implementations, one or both of the interior andexterior surfaces may have an optical coating disposed thereon.Combinations of optical coatings are possible for each of the interiorand exterior surfaces. In some implementations, the second end layer 110includes an interior surface covering a second opening 124 of theinternal cavity 116 adjacent the second end layer 110. The second endlayer 110 also includes an exterior surface opposite the interiorsurface. In these implementations, one or both of the interior andexterior surfaces have an optical coating disposed thereon. Combinationsof optical coatings are possible for each of the interior and exteriorsurfaces.

The stack of layers 104 may include interfaces 126 between adjacentlayers of the stack. In some implementations, an interface 126 betweenat least one pair of adjacent layers in the stack 104 includes a directbond between the pair. In some implementations, an interface 126 betweenat least one pair of adjacent layers in the stack 104 includes anadhesion layer. The adhesion layer may assist in bonding the pair ofadjacent layers to each other. In some variations, the adhesion layerincludes silicon oxide. For example, the stack of layers 104 mayalternate between layers formed of silicon and layers formed ofborosilicate glass. An adhesion layer of silicon oxide (e.g., SiO₂,SiO_(x), etc.) may be present at one or more interfaces 126 tofacilitate the bonding of the stack of layers 104 to each other duringmanufacture. The adhesion layer may be formed on one or both sides ofeach layer of silicon to define a bonding surface that can form siloxanebonds with the layers of borosilicate glass, such as through an anodicbonding process or a contact bonding process.

The stack of layers 104 may be bonded to each other using, for example,an anodic bonding process, a contact bonding process, a glass fritbonding process, or some other type of bonding process. Combinations ofbonding processes are also possible. In some variations, a contactbonding process is the final bonding process that is used in bonding thelayers. For example, the first end layer 106 and the intermediate layers114 may be bonded to each other using an anodic bonding process. Acontact bonding process may then be used to bond the second end layer110 to the intermediate layers 114, thereby hermetically sealing thevapor or a source of the vapor in the internal cavity 116. As part ofthe contact bonding process, the second end layer 110 may be positionedto cover an opening of the internal cavity 116 after the internal cavity116 is filled with the vapor or contains the source of the vapor. Inmany instances, the internal cavity 116 is evacuated prior to receivingthe vapor or the source of the vapor.

The contact-bonding process may include processes to chemically alter afirst bonding surface of a first layer (e.g., a last intermediate layer)and a second bonding surface of a second layer (e.g., the second endlayer 110). Before alteration, one or both of the first and secondbonding surfaces may have a surface roughness, Ra, no greater than athreshold surface roughness. For example, the threshold surfaceroughness may be 1 nm. In some instances, the first and second bondingsurfaces are planar surfaces. The contact bonding process includesaltering the first and second bonding surfaces to include, respectively,a first plurality of hydroxyl ligands and a second plurality of hydroxylligands. The altered surfaces are then contacted together to formmetal-oxygen bonds (e.g., siloxane bonds) that create a bond between thefirst and second layers (e.g., a seal). The metal-oxygen bonds may formwhen the first plurality of hydroxyl ligands react with the secondplurality of hydroxyl ligands during contact.

In some implementations, altering the first and second bonding surfacesincludes activating one or both of the first and second bonding surfacesby exposing their respective surfaces to a plasma. Such exposure mayincrease a surface energy of the first and second bonding surfaces andchemically prepare the first and second surfaces for subsequent contactbonding. Further chemical preparation may occur by washing, afteractivation, one or both of the first and second bonding surfaces inwater (e.g., deionized water) or a basic aqueous solution. Contact withthe water or basic aqueous solution may coordinate metal atoms on thefirst and second bonding surfaces with hydroxyl ligands.

In some implementations, a contact bond may be formed using metal-oxygenbonds between adjacent layers in the stack of layers 104. For example,the intermediate layers 114 may extend in sequence from a firstintermediate layer to a last intermediate layer. The first intermediatelayer may be bonded to the first end layer 106, and the second end layer110 may be positioned adjacent the last intermediate layer. In somevariations, a contact bond may be formed between the second end layer110 and the last intermediate layer using siloxane bonds (i.e.,Si—O—Si). A reaction of the contact-bond formation process may berepresented by Equation (1):Si—OH+HO—Si ⇄Si—O—Si+H₂O  (1)The reaction may be allowed in cases where, for example, the second endlayer 110 is formed of silicon oxide and the last intermediate layer isformed of silicon. However, other silicon-containing materials arepossible. The reaction is reversible, so in some instances, it isdesirable to remove the water molecules generated from the reaction.Otherwise, the newly-formed siloxane bonds are at risk in beinghydrolyzed back into silanol bonds (i.e., Si—OH).

Water molecules generated by contact-bonding can be removed by reactionswhose products are solid at room temperature. In some implementations,the water molecules are reacted with the vapor in the example vapor cell100. For example, the vapor may be a gas of cesium atoms, and the watermolecules may be reacted with a portion of the gas to form solids, suchas Cs₂O (T_(melt)≅340° C.), CsOH (T_(melt)≅272° C.), or CsH(T_(melt)≅170° C.). In some implementations, the water molecules arereacted with a desiccant material that resides in the internal cavity116 (e.g., as a coating, a dotted mass, etc.). The desiccant materialmay be inert to the vapor in the vapor cell. For example, the vapor maybe a gas of diatomic halogen molecules (e.g., chlorine gas), and thewater molecules may be reacted with an anhydrous chloride salt (e.g.,LaCl₃) to form products, such as hydrated salts or oxyhydroxidecompounds (e.g., LaCl₃.xH₂O, LaOCl, etc.).

Although the formation reaction above is presented in the context ofsilicon as the participating metal atoms, other metal atoms arepossible. For example, if a first layer is formed of aluminum oxide(e.g., single-crystal sapphire) and a second, adjacent layer is alsoformed of aluminum oxide (e.g., Al₂O₃ polycrystalline ceramic), thecontact-bond formation process may utilize aluminum as the metal atomand form oxo-aluminum bonds (e.g., Al—O—Al). Mixtures of metals are alsopossible. For example, if the first layer is formed of zirconium oxideand the second, adjacent layer is formed of magnesium oxide, thecontact-bond process may utilize zirconium and magnesium as the metalatoms and form zirconium-oxo-magnesium bonds.

In general, for a first layer that includes a first metal, M₁, and asecond, adjacent layer that includes a second metal, M₂, a reaction ofthe contact-bond formation process may be represented by Equation (2):M₁-OH+HO-M₂⇄M₁-O-M₂+H₂O  (2)Here, a hydroxyl ligand (i.e., OH) is coordinated to each of the metalatoms, M₁ and M₂, and the hydroxyl ligands condense into an oxo ligand(O) during formation of the metal-oxygen bond (i.e., M₁-O-M₂). A watermolecule is liberated as a by-product of this condensation process.Although the reaction suggests a single hydroxyl ligand per metal atom,other numbers of hydroxyl ligands may be coordinated to each of themetal atoms, M₁ and M₂.

In many variations, the condensation of hydroxyl ligands occurs at roomtemperature upon contact of the first and second adjacent layers (ormating surfaces thereof). However, in some variations, heat may beapplied to initiate and/or complete the formation of the contact bond.The heat may also strengthen the contact bond. For example, heat may beapplied to one or both of the first and second adjacent layers toincrease their respective temperatures to a processing temperature. Theprocessing temperature may facilitate formation of the contact bond. Insome variations, the processing temperature is no greater than 250° C.In some variations, the processing temperature is no greater than 120°C. In some variations, the processing temperature is no greater than 75°C. Contact bonding processes are further described in U.S. Pat. No.10,859,981 entitled “Vapor Cells Having One or More Optical WindowsBonded to a Dielectric Body.”

In some implementations, contact bonding processes may provide certainadvantages over anodic bonding processes, for example, in applicationsthat are sensitive to the purity of the vapor in the internal cavity116. The process of forming an anodic bond can, in some instances,introduce undesirable gases into the internal cavity 116, such as bygenerating one or more volatile chemical species. These gases mayproduce a background vapor pressure that can have a deleterious effecton the vapor (e.g., broadens the Rydberg transition of atoms ormolecules in the vapor). The process of forming a contact bonding doesnot rely on the large electric fields and high temperatures common toanodic bonding and thereby mitigates (or eliminates entirely) theintroduction of undesirable gases into the internal cavity 116. If abackground vapor pressure does result from the contact bonding process,this pressure is notably less than the pressure of the desired vapor inthe internal cavity 116 and can still allow for a high purity vapor.

In some implementations, the example vapor cell 100 is a first vaporcell that can be mechanically connected (e.g., interlocked) with othervapor cells. The mechanical connections between the vapor cells can beformed by mating complementary mechanical interfaces of respective vaporcells. For example, the first set of tabs 118 a may define a firstmechanical interface that is configured to interlock with a mechanicalinterface of a second vapor cell. Similarly, the second set of tabs 118b may define a second mechanical interface that is configured tointerlock with a mechanical interface of a third vapor cell. In someimplementations, the example vapor cell 100 is part of a plurality ofsuch cells. In these implementations, the first and second subsets oftabs 118 a, 118 b of each vapor cell are interlocked with, respectively,the second subset of tabs 118 b of a first vapor cell and the firstsubset of tabs 118 a of a second, different vapor cell.

FIG. 1G presents a schematic diagram, in perspective view, of threeinstances 100 a, 100 b, 100 c of the example vapor cell 100 of FIG. 1Ain which two instances 100 a, 100 b are coupled to each other. The firstinstance 100 a includes a set of tabs 150 extending outward from thesecond subset of layers 114 b but not the first subset of layers 114 a.The first and second subset of layers 114 a, 114 b alternate in sequenceand thus create gaps in the set of tabs 150 (e.g., gaps between adjacenttabs) capable of receiving tabs from another instance, such as the thirdinstance 100 c. The gaps may be dimensioned to allow a sliding fit, suchas a slip fit, with these tabs. The third instance 100 c includes a setof tabs 152 that, converse to the first instance 100 a, extends outwardfrom the first subset of layers 114 a but not the second subset oflayers 114 b. This inverse arrangement allows the set of tabs 152 tointerlock with the set of tabs 150 during engagement. As such, the setsof tabs 150, 152 define respective interlockable mechanical interfacesfor the first and third instances 100 a, 100 c. An arrow 154 indicates amotion of the third instance 100 c to engage or disengage the set oftabs 152 with the set of tabs 150 from the first instance 100 a, therebycoupling or uncoupling the two instances 100 a, 100 c. In somevariations, the sets of tabs 150, 152 may be bonded to each other afterbeing interlocked by using, for example, an anodic bonding process, acontact bonding process, a glass frit bonding process, or some othertype of bonding process (e.g., mechanical bonding, such as with a screwor pin formed of dielectric material).

Instances of the example vapor cell 100 may be interlocked with eachother to form a multi-dimensional array or tiled pattern. In somevariations, instances of the example vapor cell 100 are interlocked witheach other to form a patterned subassembly. For example, FIG. 2Apresents a schematic diagram, in perspective view, of five instances ofthe example vapor cell 100 of FIG. 1A that are interlocked with eachother to form a “tee” subassembly. As another example, FIG. 2B presentsa schematic diagram, in perspective view, of eight instances of theexample vapor cell 100 of FIG. 1A that are interlocked with each otherto form an octagon subassembly. Other arrangements are possible for thepatterned subassembly. In further variations, the patternedsubassemblies define a repeatable motif for a larger assembly or tiledpattern (e.g., a mosaic). For example, FIG. 3A presents a schematicdiagram, in perspective view, of five instances of the “tee” subassemblyof FIG. 2A that are interlocked with each other to form an arcuateassembly. As another example, FIG. 3B presents a schematic diagram, intop view, of four instances of the octagon subassembly of FIG. 2B thatare interlocked with each other to form tiled pattern. Other largeassemblies or tiled patterns are possible. For example, an assembly ofinterlocked vapor cells may be connected to form a variety of spatialarrangements (e.g., ordered or disordered arrays) in two or threespatial dimensions.

A tiled pattern or array of interlocked vapor cells may have severaladvantages, especially for imaging applications. During operation, theinterlocked vapor cells may measure a target electromagnetic field thattraverses a volume occupied by the interlocked vapor cells. As part ofthis measurement, a laser may generate optical signals that interactwith the vapor in the internal cavities of one or more interlocked vaporcells. Such interaction allows the target electromagnetic field to bereadout by the one or more interlocked vapor cells. However, whenarranged in a tiled pattern or array, the power used by the laser toreadout a target electromagnetic field can be efficiently distributed toeach individual vapor cell (e.g., such as through a fiber optic cable orgroup of such cables). This distribution allows only the necessarysampling of the target electromagnetic field during readout. Moreover,after interacting with the vapor(s), the optical signals can be detectedusing individual optical detectors (e.g., photodiodes). These opticaldetectors may then produce respective detector signals (e.g., electricalsignals) that can be processed using SoCs or FPGAs. In some cases,modulation techniques can be implemented to achieve high signal-to-noiseratios for each interlocked vapor cell (or channel).

The interlocked vapor cells, when arranged in a tiled pattern or array,may also present a lower amount of material to the targetelectromagnetic field, thereby maximizing a group transparency of theinterlocked vapor cells. Furthermore, unlike implementations that relyon a large single vapor cell, a tiled pattern or array can be repairedat a granular level, for example, by replacing one or more individualinterlocked vapor cells. This capability avoids having to replace theentire tiled pattern or array should its operational performance becomeimpaired due to a malfunctioning portion. Additionally, the detectorsignals generated during operation of the tiled pattern or array can beequalized to zero-out interference from certain directions. Byequalizing these signals, a better detection of desired signals can beobtained. Moreover, equalizing the signals may allow the tiled patternor array to select a direction along which the target electromagneticfield is measured. Such selection or ‘steering’ is analogous to thesteering of a multi-element, beam-forming metal antenna. It will beappreciated that, due to its granular configuration, the tiled patternor array can also compensate for a malfunctioning or dead cell in thetiled pattern or array. For example, the arrayed or tiled arrangementcan oversample signals coming from different directions and thus remainrobust to aging. Other advantages are also possible.

In some implementations, at least one of the intermediate layers 114includes a plurality of holes 128 between the internal cavity 116 and anouter perimeter of the intermediate layer 114 (e.g., as shown in FIGS.1E and 1F). The plurality of holes 128 may extend partially through orcompletely through the intermediate layer 114. In these implementations,the example vapor cell 100 may be configured to detect a targetradiation and each of the plurality of holes 128 may have a largestdimension no greater than a wavelength of the target radiation. Forexample, the target radiation may have a wavelength of at least 0.3 mmand each of the plurality of holes 128 may have a largest dimension nogreater than 0.3 mm. In some implementations, one or more of theplurality of holes 128 may include a source of the vapor therein. Inthese embodiments, the intermediate layer 114 may define a channel (orpart thereof) that fluidly couples the one or more holes 128 to theinternal cavity 116.

FIGS. 4A and 4B present electromagnetic simulations, includingabsorptive effects, of two example configurations of vapor cells. Theexample vapor cells represented in FIGS. 4A and 4B may include featuresthat are analogous to the example vapor cell 100 of FIG. 1A. In theexample simulations represented in FIGS. 4A and 4B, the incidentelectric field is simulated to have an amplitude of 1 V m⁻¹. The exampleconfiguration of FIG. 4A corresponds to a vapor cell in which aplurality of holes is present in each intermediate layer (e.g., betweenthe internal cavity and an outer perimeter of the intermediate layer).By way of comparison, the example configuration of FIG. 4B correspondsto a vapor cell in which the plurality of holes is absent (e.g., thebody has solid walls). In these example configurations, the outergeometry has a square cross-section. However, other shapes are possible(e.g., a circular cross-section). The outer dimensions are square with 1cm sides. The diameter of the internal cavity, which is cylindrical, is7 mm. In FIG. 4A, material has been removed from the intermediate layersto create perforations therein (e.g., holes, cavities, etc.). Suchperforations may allow the vapor cell to be engineered for an RFelectric field inside the internal cavity. The thickness of the layersand the hole patterns used for each layer may be changed as needed, suchas to match a target radiation for sensing. The electric fielddistribution in the perforated configuration is more uniform andunperturbed than the non-perforated configuration. In addition, thefirst dipole resonance of the structure has its peak at higher frequencyin the perforated configuration than the non-perforated configuration.

Each of the vapor cells in FIGS. 4A and 4B may be configured to detect atarget radiation having a wavelength, λ. Even though each vapor cell hasa dimension of about λ/15 at 2 GHz, differences can be observed in thefield distributions, e.g., the electric field uniformity inside thevapor cell volume. These differences can be amplified at the largerfrequencies. For example, the first dipole resonance of the vapor cellwill start to be significant for the configuration with non-perforatedwalls unless the λ/15 dimensions are maintained, which makes the vaporcell fragile while at the same time reducing the sensing volume. In somecases, perforated walls can allow high performance in a larger vaporcell, which will increase the signal to noise (e.g., through more atomicvapor) and make it easier to build an array of vapor cells. The largersized vapor cell can also help in manufacturing large quantities ofvapor cells that conform to certain specifications. For higherfrequencies (e.g., greater than 40 GHz), the vapor cells can also befabricated from single wafers of glass and silicon using the sameprinciples. In some examples, the radar cross-section for the squarecell is 8×10⁻⁷ m² (about 0.008 the geometric cross-section) at 2 GHzwhile the radar cross-section for the perforated configuration is 4×10⁻⁷m² (about 0.004 the geometric cross-section) at 2 GHz.

FIG. 5 presents electromagnetic simulations of electric fielddistributions inside various example configurations of vapor cellshaving perforated walls. Different configurations of hole patterns,e.g., in panels (2)-(4), are compared to that of a cubic cell with noperforations in the walls, e.g., panel (1). In the example simulationsrepresented in FIG. 5, the incident electric field has unit amplitude.As shown, the vapor cells with perforated walls have narrower electricfield amplitude distributions than the non-perforated control vaporcell. The average field illustrated in panels (2) and (4) has less than1% variation through the vapor cell in the interaction region shown(hatched region in the center of each cross-section). The vapor cellsare configured to detect a target radiation having a wavelength, λ, andare simulated for frequencies where the dimensions (e.g., side lengths)are about λ/3-λ/4.

In some implementations, manufacturing the example vapor cell 100includes obtaining a stack of layers 104 that are bonded to each other,such as through anodic bonding, contact bonding, glass frit bonding, orsome other type of bonding. Combinations of bonding processes are alsopossible. The stack of layers 104 includes a first end layer 106 andintermediate layers 114 extending in sequence from a first intermediatelayer to a last intermediate layer. The first end layer 106 is formed ofa first type of dielectric material that is optically transparent, andthe first intermediate layer is bonded to the first end layer 106. Theintermediate layers 114 include a first subset of layers 114 a and asecond subset of layers 114 b. The first subset of layers 114 a isformed of the first type of dielectric material and the second subset oflayers 114 b is formed of a second, different type of dielectricmaterial. An internal cavity 116 extends through the intermediate layers114 and includes an opening 122 defined by a surface of the lastintermediate layer.

The manufacturing process may also include disposing a vapor or a sourceof the vapor into the internal cavity 116 and bonding a second end layer110 of the stack of layers 104 to the last intermediate layer to sealthe vapor or the source of the vapor in the internal cavity 116. Thesecond end layer 110 is formed of dielectric material that, in somevariations, is the first type of dielectric material. The stack oflayers 104, when including the bonded second end layer 110, defines abody 102 of the example vapor cell 100. Moreover, the first end layer106 resides at a first end of the body 102 and the second end layer 110resides at a second, opposite end of the body 102.

In some implementations, obtaining the stack of layers 104 includesalternately bonding layers of the first and second subset of layers 114a, 114 b to each other to form the intermediate layers 114. Obtainingthe stack of layers 104 may also include bonding the first intermediatelayer to the first end layer 106, such as through anodic bonding,contact bonding, glass frit bonding, or some other type of bonding. Infurther implementations, obtaining the stack of layers 104 includesremoving material from a first wafer formed of the first type ofdielectric material to fabricate the first end layer, one or more of thefirst subset of layers, or both. Obtaining the stack of layers 104 mayalso include removing material from a second wafer formed of the secondtype of dielectric material to fabricate one or more of the secondsubset of layers. Removing material may include machining material froma wafer with a laser, etching material from a wafer, or both. Theprocess of etching may involve one or both of a dry or wet etchingprocess. Other types of subtractive processes are possible for removingmaterial (e.g., ablation, grinding, polishing, etc.).

In some implementations, bonding the second end layer 110 includesaltering the surface of the last intermediate layer to include a firstplurality of hydroxyl ligands. Bonding the second end layer 110 alsoincludes altering a surface of the second end layer to include a secondplurality of hydroxyl ligands. In these implementations, the alteredsurfaces may be contacted to each other to form a seal around theopening of the internal cavity. The seal is defined by metal-oxygenbonds formed by reacting the first plurality of hydroxyl ligands withthe second plurality of hydroxyl ligands during contact of the twoaltered surfaces, such as described above in relation to Equations (1)and (2).

FIGS. 1A-5 illustrate the example vapor cell 100 in the context of asingle internal cavity 116. However, other numbers and types of cavitiesare possible. In some implementations, the stack of layers 104 defines aplurality of internal cavities 116, each of which, corresponds to avapor subcell (or unit cell). In some implementations, the stack oflayers 104 may define one or more holes passing therethrough (e.g., athrough-hole). The one or more holes may extend between a pair ofopenings defined by exterior surfaces of the first and second end layers106, 110. In some implementations, the stack of layers 104 defines boththe plurality of internal cavities 116 and the one or more holes. Theplurality of internal cavities 116 and the one or more holes may bearranged according to a pattern or array, such as shown in FIGS. 2A-2Band 3A-3B. In implementations having one or both of the plurality ofcavities and the one or more holes, the stack of layers 104 may beconfigured without the first and seconds set of tabs 118 a, 118 b.

In some implementations, the plurality of cavities 116 and the one ormore holes may be arranged according to a two-dimensional lattice (e.g.,a square lattice, a rectangular lattice, a hexagonal lattice, arhombohedral lattice, a centered lattice, an oblique lattice, etc.). Inthese implementations, the two-dimensional lattice may include firstlattice sites for the plurality of cavities 116. The first lattice sitesmay each be occupied by an internal cavity 116. However, in somevariations, only a portion of the first lattice sites is occupied. Thetwo-dimensional lattice may also include second lattice sites for theone or more holes. The second lattice sites may each be occupied by ahole. However, in some variations, only a portion of the second latticesites is occupied. It will be appreciated that the occupancy (orvacancy) of the first and second lattice sites may be selected to allowthe internal cavities 116 and holes to be arranged in a desired patternor array. Such selection may also allow the stack of layers 104 todefine a desired shape for the body 102.

FIG. 6A presents a schematic diagram of an example vapor cell 600 havingan array of cavities therein 612. The schematic diagram illustrates theexample vapor cell 600 in the context of a unit cell 602 and as part ofa tiled pattern 604 of vapor cells. The schematic diagram alsoillustrates a cross-section 606 of the example vapor cell 600. Theexample vapor cell 600 includes a body 608 defined by a stack of layers610 bonded to each other. The stack of layers 610 defines the array ofcavities 612, which includes a first subset of cavities 612 a and asecond subset of cavities 612 b. The first subset of cavities 612 aextends through intermediate layers 614 of the stack of layers 610, andthe second subset of cavities 612 b extends entirely through the stackof layers 610. The first subset of cavities 612 a may be analogous tothe internal cavity 116 described in relation to FIGS. 1A-5, and thesecond subset of cavities 612 b may each correspond to a through-hole. Avapor or a source of the vapor is disposed in each of the first subsetof cavities 612 a.

The first and second subsets of cavities 612 a may each share a commonshape, as shown in FIG. 6A. However, in some cases, one or more of thefirst subset of cavities 612 a can have a different shape. In somecases, one or more of the second subset of cavities 612 b can also havea different shape. In many variations, the first and second subset ofcavities 612 a, 612 b run along respective axes that are perpendicularto the stack of layers 610, although other orientations are possible(e.g., canted axes). Furthermore, although FIG. 6A depicts the firstsubset of cavities 612 a as having a cylindrical shape and second subsetof cavities 612 b as having a square tubular shape, other shapes arepossible (e.g., hexagonal, ellipsoidal, spherical, etc.). For example,each intermediate layer 614 may include a through-hole that defines aportion of a cavity through the intermediate layer 614. One or moreintermediate layers 614 may then be selectively configured such that theintermediate layers 614, when stacked, define a target three-dimensionalvolume for the cavity (e.g., a sphere, a frustrum, an inclinedparallelepiped, etc.). In some variations, at least two adjacentintermediate layers 614 have respective through-holes that differ,relative to each other, in one or both of shape and size. In somevariations, two or more intermediate layers 614 (e.g., adjacentintermediate layers) have different thicknesses. Other configurablefeatures are possible (e.g., angles for an inner perimeter surface thatencircles the through-hole of an intermediate layer).

The stack of layers 610 includes a first end layer 616 at a first end618 of the body 608. The first end layer 616 is formed of a first typeof dielectric material that is optically transparent. The stack oflayers 610 also includes a second end layer 620 at a second, oppositeend 622 of the body 608. The second end layer 620 is formed ofdielectric material. The intermediate layers 614 of the stack of layers610 are positioned between the first and second end layers 616, 620 andinclude a first subset of layers 614 a formed of the first type ofdielectric material and a second subset of layers 614 b formed of asecond, different type of dielectric material. FIG. 6A depicts the firstand second subsets of layers 614 a, 614 b as each being only a singlelayer. However, other numbers of layers are possible for one or both ofthe first and second subsets of layers 614 a, 614 b.

In some variations, such as shown in FIG. 6A, the stack of layers 610includes a first set of tabs 624 a extending outward from the firstsubset of layers 614 a at a first location 626 a of an exteriorperimeter 628 of the body 608. The stack of layers 610 may also includea second set of tabs 624 b extending outward from the second subset oflayers 614 b at a second location 626 b of the exterior perimeter 628 ofthe body 608. The first and second tabs 624 a, 624 b may function as,respectively, first and second mechanical interfaces for interlockingwith another, similarly-configured vapor cell (e.g., a second instanceof the example vapor cell 600). However, in some variations, the firstand second sets of layers 614 a, 614 b have no tabs extending therefrom.FIG. 6B presents a schematic diagram of the example vapor cell of FIG.6A, but in which intermediate layers of the example vapor cell (e.g.,the first and second sets of layers 614 a, 614 b) lack tabs extendingoutward therefrom.

In certain instances, each of the first subset of cavities 612 a isencircled by a side wall 630 formed by the stack of layers 610. The sidewall 630 includes a corner that defines part of the exterior perimeter628. In such instances, the first location 626 a is a first corner ofthe exterior perimeter 628 and the second location 626 b is a second,different corner of the exterior perimeter 628. However, the first andsecond locations 626 a, 626 b can be elsewhere on the exterior perimeter628. For example, the side wall 630 may include a flat surface thatdefines part of the exterior perimeter 628 (e.g., a center of anexterior side). The first location 626 a may then be a first flatsurface of the exterior perimeter 628 and the second location 626 b is asecond, different flat surface of the exterior perimeter 628.

The array of cavities 612 may be arranged according to a two-dimensionallattice. In some cases, the two-dimensional lattice includes first andsecond lattice sites for, respectively, the first and second subsets ofcavities 612 a, 612 b. For example, the two-dimensional lattice may be asquare lattice having first and second lattice sites that alternate insequence along row and column directions. In this arrangement, the arrayof cavities 612 are disposed at respective locations of the squarelattice and the first and second subset of cavities 612 a, 612 balternate in sequence along rows and columns of the square lattice. Insome cases, the first and second lattice sites are partially occupied bythe first and second subset of cavities 612 a, 612 b. For example, asshown in FIG. 6, four of the first lattice sites may be occupied byrespective cavities of the first subset 612 a and one second latticesite may be occupied by a respective cavity of the second subset 612 b.This configuration may allow the body 608 of the example vapor cell 600to assume a plus (+) shape. However, other arrangements are possible.For example, the array of cavities 612 in the example vapor cell 600 ofFIG. 6B is arranged to define a butterfly-like shape for the body 608.

In some variations, the side wall 630 has four exterior surfaces thatdefine a square cross-section of the side wall 630. In these variations,each of the second subset of cavities 612 b has a square cross-section.In some variations, the side wall 630 has one or more exterior surfacesthat define an exterior unit-cell perimeter 632 of the cavity. In FIGS.6A and 6B, only a single exterior unit-cell perimeter 632—represented bya dashed line—is shown for clarity. For one or more cavities of thefirst subset 612 a, at least one intermediate layer 614 includes aplurality of holes between the one or more cavities and their respectiveexterior unit-cell perimeters 632. The plurality of holes may beanalogous to those described in relation to FIGS. 1A-1B and 4A-5. Forexample, the plurality of holes may extend through the at least oneintermediate layer 614. As another example, the example vapor cell 600may be configured to detect a target radiation and each of the pluralityof holes has a largest dimension no greater than a wavelength of thetarget radiation (e.g., a wavelength of at least 0.3 mm). In certaincases, one or more of the plurality of holes adjacent the one or morecavities may include a source of the vapor therein. In theseembodiments, the at least one intermediate layer 614 may define achannel (or part thereof) that fluidly couples the one or more holes tothe one or more cavities.

In some aspects of what is described, a vapor cell may be described bythe following examples:

-   Example 1. A vapor cell, comprising:    -   a body defined by a stack of layers bonded to each other, the        stack of layers comprising:        -   a first end layer at a first end of the body, the first end            layer being formed of a first type of dielectric material            that is optically transparent,        -   a second end layer at a second, opposite end of the body,            the second end layer being formed of dielectric material,        -   intermediate layers between the first and second end layers            and formed of dielectric material, the intermediate layers            defining an internal cavity that extends through the body            between the first end layer and the second end layer; and    -   a vapor or a source of the vapor disposed in the internal        cavity.-   Example 2. The vapor cell of example 1, wherein the second end layer    is formed of the first type of dielectric material.-   Example 3. The vapor cell of example 1 or example 2, wherein the    intermediate layers comprise:    -   a first subset of layers formed of the first type of dielectric        material; and    -   a second subset of layers formed of a second, different type of        dielectric material.-   Example 4. The vapor cell of example 3, wherein the layers are    ordered in the stack to alternate between the first type of    dielectric material and the second type of dielectric material.-   Example 5. The vapor cell of example 3 or example 4, wherein the    second type of dielectric material is silicon.-   Example 6. The vapor cell of example 1 or any one of examples 2-5,    wherein the first type of dielectric material comprises silicon    oxide.-   Example 7. The vapor cell of example 1 or any one of examples 2-6,    wherein the stack of layers comprises:    -   a first set of tabs extending outward from the intermediate        layers on a first exterior side (or location) of the body; and    -   a second set of tabs extending outward from the intermediate        layers on a second exterior side (or location) of the body.-   Example 8. The vapor cell of example 7,    -   wherein the vapor cell is a first vapor cell;    -   wherein the first set of tabs defines a first mechanical        interface that is configured to interlock with a second vapor        cell (e.g., a second instance of the first vapor cell); and    -   wherein the second set of tabs defines a second mechanical        interface that is configured to interlock with a third vapor        cell (e.g., a third instance of the first vapor cell).-   Example 9. The vapor cell of example 7 or example 8,    -   wherein each tab of the first set is an integral part of the        intermediate layer from which it extends; and    -   wherein each tab of the second set is an integral part of the        intermediate layer from which it extends.-   Example 10. The vapor cell of example 7 or any one of examples 8-9,    wherein the first and second sets of tabs are aligned with    respective centers of the first and second exterior sides.-   Example 11. The vapor cell of example 7 or any one of examples 8-10,    -   wherein the intermediate layers comprise a first subset of        layers and a second subset of layers that alternate in sequence        along the stack of layers;    -   wherein each tab of the first set extends outward from a        respective layer in the first subset of layers; and    -   wherein each tab of the second set extends outward from a        respective layer in the second subset of layers.-   Example 12. A plurality of vapor cells, each corresponding to the    vapor cell of example 7 or any one of examples 8-11, in which the    first and second sets of tabs of each vapor cell are interlocked    with, respectively, the second set of tabs of a first vapor cell and    the first set of tabs of a second, different vapor cell.-   Example 13. The vapor cell of example 1 or any one of examples 2-12,    -   wherein the body has four exterior sides defining a square        cross-section of the body, the four exterior sides comprising        first and second exterior sides; and    -   wherein the stack of layers comprises:        -   a first set of tabs extending outward from the intermediate            layers on the first exterior side of the body, and        -   a second set of tabs extending outward from the intermediate            layers on the second exterior side of the body.-   Example 14. The vapor cell of example 13, wherein the first and    second exterior sides are adjacent to each other.-   Example 15. The vapor cell of example 13, wherein the first and    second exterior sides are opposite each other.-   Example 16. The vapor cell of example 13 or any one of examples    14-15,    -   wherein the four exterior sides comprise third and fourth        exterior sides; and    -   wherein the stack of layers comprises:        -   a third set of tabs extending outward from the intermediate            layers on the third exterior side of the body, and        -   a fourth set of tabs extending outward from the intermediate            layers on the fourth exterior side of the body.-   Example 17. The vapor cell of example 16,    -   wherein each tab of the third set is an integral part of the        intermediate layer from which it extends; and    -   wherein each tab of the fourth set is an integral part of the        intermediate layer from which it extends.-   Example 18. The vapor cell of example 16 or example 17, wherein the    first, second, third, and fourth sets of tabs are aligned with    respective centers of the first, second, third, and fourth exterior    sides.-   Example 19. The vapor cell of example 16 or any one of examples    17-18,    -   wherein the intermediate layers comprise a first subset of        layers and a second subset of layers that alternate in sequence        along the stack of layers;    -   wherein each tab of the first and third sets extends outward        from a respective layer in the first subset of layers; and    -   wherein each tab of the second and fourth sets extends outward        from a respective layer in the second subset of layers.-   Example 20. The vapor cell of example 1 or any one of examples 2-19,    -   wherein the first end layer comprises:        -   an interior surface covering a first opening of the internal            cavity adjacent the first end layer, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 21. The vapor cell of example 20, wherein the interior    surface of the first end layer comprises a reflective coating    disposed thereon.-   Example 22. The vapor cell of example 20 or example 21, wherein the    exterior surface of the first end layer comprises a reflective    coating disposed thereon.-   Example 23. The vapor cell of example 20 or any one of examples    21-22, wherein the interior surface of the first end layer comprises    an anti-reflective coating disposed thereon.-   Example 24. The vapor cell of example 20 or any one of examples    21-23, wherein the exterior surface of the first end layer comprises    an anti-reflective coating disposed thereon.-   Example 25. The vapor cell of example 1 or any one of examples 2-24,    -   wherein the second end layer comprises:        -   an interior surface covering a second opening of the            internal cavity adjacent the second end layer, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 26. The vapor cell of example 25, wherein the interior    surface of the second end layer comprises a reflective coating    disposed thereon.-   Example 27. The vapor cell of example 25 or example 26, wherein the    exterior surface of the second end layer comprises a reflective    coating disposed thereon.-   Example 28. The vapor cell of example 25 or any one of examples    26-27, wherein the interior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 29. The vapor cell of example 25 or any one of examples    26-28, wherein the exterior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 30. The vapor cell of example 1 or any one of examples 2-29,    wherein at least one of the intermediate layers comprises a    plurality of holes between the internal cavity and an outer    perimeter of the intermediate layer.-   Example 31. The vapor cell of example 30, wherein the plurality of    holes extends through the intermediate layer.-   Example 32. The vapor cell of example 30 or example 31,    -   wherein the vapor cell is configured to detect a target        radiation; and    -   wherein each of the plurality of holes has a largest dimension        no greater than a wavelength of the target radiation.-   Example 33. The vapor cell of example 32, wherein the target    radiation has a wavelength of at least 0.3 mm.-   Example 34. The vapor cell of example 1 or any one of examples 2-33,    wherein an interface between at least one pair of adjacent layers in    the stack comprises an adhesion layer.-   Example 35. The vapor cell of example 34, wherein the adhesion layer    comprises silicon oxide.-   Example 36. The vapor cell of example 1 or any one of examples 2-35,    -   wherein each intermediate layer comprises a through-hole that        defines a portion of the internal cavity through the        intermediate layer; and    -   wherein at least two adjacent intermediate layers have        respective through-holes that differ, relative to each other, in        one or both of shape and size.

In some aspects of what is described, a method of manufacturing a vaporcell may be described by the following examples:

-   Example 37. A method of manufacturing a vapor cell, the method    comprising:    -   obtaining a stack of layers bonded to each other and comprising:        -   a first end layer formed of a first type of dielectric            material that is optically transparent, and        -   intermediate layers formed of dielectric material and            extending in sequence from a first intermediate layer to a            last intermediate layer, wherein:            -   the first intermediate layer is bonded to the first end                layer, and            -   an internal cavity extends through the intermediate                layers and comprises an opening defined by a surface of                the last intermediate layer;    -   disposing a vapor or a source of the vapor into the internal        cavity; and    -   bonding a second end layer of the stack of layers to the last        intermediate layer to seal the vapor or the source of the vapor        in the internal cavity, the second end layer formed of        dielectric material;    -   wherein the stack of layers, when comprising the bonded second        end layer, defines a body of the vapor cell, the first end layer        at a first end of the body and the second end layer at a second,        opposite end of the body.-   Example 38. The method of example 37, wherein bonding the second end    layer comprises covering the opening of the internal cavity with the    second end layer to enclose the vapor or the source of the vapor in    the internal cavity.-   Example 39. The method of example 37 or example 38, wherein the    second end layer is formed of the first type of dielectric material.-   Example 40. The method of example 37 or any one of examples 38-39,    wherein the first type of dielectric material comprises silicon    oxide.-   Example 41. The method of example 37 or any one of examples 38-40,    wherein obtaining the stack of layers comprises:    -   removing material from a first wafer formed of the first type of        dielectric material to fabricate the first end layer, the second        end layer (when combined with Example 39), or both; and    -   removing material from a second wafer formed of dielectric        material to fabricate the intermediate layers.-   Example 42. The method of example 37 or any one of examples 38-40,    wherein the intermediate layers comprise:    -   a first subset of layers formed of the first type of dielectric        material; and    -   a second subset of layers formed of a second, different type of        dielectric material.-   Example 43. The method of example 42, wherein obtaining the stack of    layers comprises:    -   alternately bonding layers of the first and second subset of        layers to each other to form the intermediate layers; and    -   bonding the first intermediate layer to the first end layer.-   Example 44. The method of example 42 or example 43, wherein    obtaining the stack of layers comprises:    -   removing material from a first wafer formed of the first type of        dielectric material to fabricate the first end layer, one or        more of the first subset of layers, the second end layer (when        combined with Example 39), or any combination thereof; and    -   removing material from a second wafer formed of the second type        of dielectric material to fabricate one or more of the second        subset of layers.-   Example 45. The method of example 42 or any one of examples 43-44,    wherein the second type of dielectric material is silicon.-   Example 46. The method of example 37 or any one of examples 38-45,    wherein bonding the second end layer comprises:    -   altering the surface of the last intermediate layer to comprise        a first plurality of hydroxyl ligands;    -   altering a surface of the second end layer to comprise a second        plurality of hydroxyl ligands; and    -   contacting the altered surfaces to form a seal around the        opening of the internal cavity, the seal defined by metal-oxygen        bonds formed by reacting the first plurality of hydroxyl ligands        with the second plurality of hydroxyl ligands during contact of        the two altered surfaces.-   Example 47. The method of example 46, wherein bonding the second end    layer comprises:    -   activating one or both of the surfaces of the last intermediate        layer and the second end layer by exposing their respective        surfaces to a plasma.-   Example 48. The method of example 37 or any one of examples 38-47,    wherein at least one of the intermediate layers comprises a    plurality of the holes between the internal cavity and an outer    perimeter of the intermediate layer.-   Example 49. The method of example 48, wherein the plurality of holes    extends through the intermediate layer.-   Example 50. The method of example 48 or example 49,    -   wherein the vapor cell, when manufactured, is configured to        detect a target radiation; and    -   wherein each of the plurality of holes has a largest dimension        no greater than a wavelength of the target radiation.-   Example 51. The method of example 50, wherein the target radiation    has a wavelength of at least 0.3 mm.-   Example 52. The method of example 37 or any one of examples 38-51,    wherein obtaining the stack of layers comprises:    -   forming an adhesion layer on one or both sides of an        intermediate layer to define respective bonding surfaces of the        intermediate layer.-   Example 53. The method of example 52, wherein the adhesion layer    comprises silicon oxide.-   Example 54. The method of example 37 or any one of examples 38-53,    wherein the stack of layers comprises:    -   a first set of tabs extending outward from the intermediate        layers on a first exterior side (or location) of the stack of        layers (or body of the vapor cell); and    -   a second set of tabs extending outward from the intermediate        layers on a second exterior side (or location) of the stack of        layers (or body of the vapor cell).-   Example 55. The method of example 54,    -   wherein the vapor cell is a first vapor cell;    -   wherein the first set of tabs defines a first mechanical        interface that is configured to interlock with a second vapor        cell (e.g., a second instance of the first vapor cell); and    -   wherein the second set of tabs defines a second mechanical        interface that is configured to interlock with a third vapor        cell (e.g., a third instance of the first vapor cell).

In some aspects of what is described, a vapor cell system may bedescribed by the following examples:

-   Example 56. A vapor cell system comprising:    -   an array of interlocked vapor cells, each interlocked vapor cell        comprising:        -   a stack of layers bonded to each other and defining a body            of the interlocked vapor cell, the stack of layers            comprising:            -   a first end layer at a first end of the body, the first                end layer being formed of a first type of dielectric                material that is optically transparent,            -   a second end layer at a second, opposite end of the                body, the second end layer being formed of dielectric                material, and            -   one or more intermediate layers between the first and                second end layers and formed of dielectric material, the                one or more intermediate layers defining an internal                cavity that extends through the stack of layers between                the first end layer and the second end layer,        -   a first mechanical interface on a first exterior side of the            body,        -   a second mechanical interface on a second exterior side of            the body, and        -   a vapor or a source of the vapor disposed in the internal            cavity;    -   wherein the first mechanical interface of at least one        interlocked vapor cell is interlocked with the second mechanical        interface of another interlocked vapor cell; and    -   wherein the second mechanical interface of the at least one        interlocked vapor cell is interlocked with the first mechanical        interface of another, different interlocked vapor cell.-   Example 57. The vapor cell system of example 56, wherein the second    end layer is formed of the first type of dielectric material.-   Example 58. The vapor cell system of example 56 or example 57,    wherein the one or more intermediate layers comprise at least one    layer formed of a second, different type of dielectric material.-   Example 59. The vapor cell system of example 56 or example 57,    wherein the one or more intermediate layers comprise:    -   a first subset of layers formed of the first type of dielectric        material, and    -   a second subset of layers formed of a second, different type of        dielectric material.-   Example 60. The vapor cell system of example 59, wherein the layers    are ordered in the stack to alternate between the first type of    dielectric material and the second type of dielectric material.-   Example 61. The vapor cell system of example 58 or any one of    examples 59-60, wherein the second type of dielectric material is    silicon.-   Example 62. The vapor cell system of example 56 or any one of    examples 57-61, wherein the first type of dielectric material    comprises silicon oxide.-   Example 63. The vapor cell system of example 56 or any one of    examples 57-62,    -   wherein the first mechanical interface comprises a first set of        tabs extending outward from the one or more intermediate layers        on the first exterior side (or location) of the body; and    -   wherein the second mechanical interface comprises a second set        of tabs extending outward from the one or more intermediate        layers on the second exterior side (or location) of the body.-   Example 64. The vapor cell system of example 63,    -   wherein each tab of the first set is an integral part of the        intermediate layer from which it extends; and    -   wherein each tab of the second set is an integral part of the        intermediate layer from which it extends.-   Example 65. The vapor cell system of example 63 or example 64,    wherein the first and second sets of tabs are aligned with    respective centers of the first and second exterior sides.-   Example 66. The vapor cell system of example 63 or any one of    examples 64-65,    -   wherein the one or more intermediate layers comprise a first        subset of layers and a second subset of layers that alternate in        sequence along the stack of layers;    -   wherein each tab of the first set extends outward from a        respective layer in the first subset of layers; and    -   wherein each tab of the second set extends outward from a        respective layer in the second subset of layers.-   Example 67. The vapor cell system of example 56 or any one of    examples 57-66, wherein the body of the interlocked vapor cell has    four exterior sides defining a square cross-section of the body, the    four exterior sides comprising the first and second exterior sides.-   Example 68. The vapor cell system of example 67, wherein the first    and second exterior sides are adjacent to each other.-   Example 69. The vapor cell system of example 67, wherein the first    and second exterior sides are opposite each other.-   Example 70. The vapor cell system of example 67 or any one of    examples 68-69,    -   wherein the four exterior sides comprise third and fourth        exterior sides; and    -   wherein each interlocked vapor cell comprises:        -   a third mechanical interface on the third exterior side of            the body, and        -   a fourth mechanical interface on the fourth exterior side of            the body.-   Example 71. The vapor cell system of example 70,    -   wherein the third mechanical interface comprises a third set of        tabs extending outward from the one or more intermediate layers        on the third exterior side of the body; and    -   wherein the fourth mechanical interface comprises a fourth set        of tabs extending outward from the one or more intermediate        layers on the fourth exterior side of the body.-   Example 72. The vapor cell system of example 71,    -   wherein each tab of the third set is an integral part of the        intermediate layer from which it extends; and    -   wherein each tab of the fourth set is an integral part of the        intermediate layer from which it extends.-   Example 73. The vapor cell system of example 71 or example 72,    wherein the third and fourth sets of tabs are aligned with    respective centers of the third and fourth exterior sides.-   Example 74. The vapor cell system of example 71 or any one of    examples 72-73,    -   wherein the one or more intermediate layers comprise a first        subset of layers and a second subset of layers that alternate in        sequence along the stack of layers;    -   wherein each tab of the third set extends outward from a        respective layer in the first subset of layers; and    -   wherein each tab of the fourth set extends outward from a        respective layer in the second subset of layers.-   Example 75. The vapor cell system of example 56 or any one of    examples 57-74,    -   wherein the first end layer comprises:        -   an interior surface covering a first opening of the internal            cavity adjacent the first end layer, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 76. The vapor cell system of example 75, wherein the    interior surface of the first end layer comprises a reflective    coating disposed thereon.-   Example 77. The vapor cell system of example 75 or example 76,    wherein the exterior surface of the first end layer comprises a    reflective coating disposed thereon.-   Example 78. The vapor cell system of example 75 or any one of    examples 76-77, wherein the interior surface of the first end layer    comprises an anti-reflective coating disposed thereon.-   Example 79. The vapor cell system of example 75 or any one of    examples 76-78, wherein the exterior surface of the first end layer    comprises an anti-reflective coating disposed thereon.-   Example 80. The vapor cell system of example 56 or any one of    examples 57-79,    -   wherein the second end layer comprises:        -   an interior surface covering a second opening of the            internal cavity adjacent the second end layer, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 81. The vapor cell system of example 80, wherein the    interior surface of the second end layer comprises a reflective    coating disposed thereon.-   Example 82. The vapor cell system of example 80 or example 81,    wherein the exterior surface of the second end layer comprises a    reflective coating disposed thereon.-   Example 83. The vapor cell system of example 80 or any one of    examples 81-82, wherein the interior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 84. The vapor cell system of example 80 or any one of    examples 81-83, wherein the exterior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 85. The vapor cell system of example 56 or any one of    examples 57-84, wherein the one or more intermediate layers comprise    at least one layer having a plurality of holes between the internal    cavity and an outer perimeter of the intermediate layer.-   Example 86. The vapor cell system of example 85, wherein the    plurality of holes extends through the intermediate layer.-   Example 87. The vapor cell system of example 85 or example 86,    -   wherein each interlocked vapor cell is configured to detect a        respective target radiation; and    -   wherein the plurality of holes associated with an interlocked        vapor cell each have a largest dimension no greater than a        wavelength of the target radiation of the interlocked vapor        cell.-   Example 88. The vapor cell system of example 87, wherein the target    radiation has a wavelength of at least 0.3 mm.-   Example 89. The vapor cell system of example 56 or any one of    examples 57-88, wherein an interface between at least one pair of    adjacent layers in the stack comprises an adhesion layer.-   Example 90. The vapor cell system of example 89, wherein the    adhesion layer comprises silicon oxide.-   Example 91. The vapor cell system of example 56 or any one of    examples 57-90,    -   wherein each intermediate layer comprises a through-hole that        defines a portion of the internal cavity through the        intermediate layer; and    -   wherein at least two adjacent intermediate layers have        respective through-holes that differ, relative to each other, in        one or both of shape and size.

In some aspects of what is described, a vapor cell may be described bythe following examples:

-   Example 92. A vapor cell, comprising:    -   a body defined by a stack of layers bonded to each other, the        stack of layers defining an array of cavities that comprises a        first subset of cavities and a second subset of cavities, the        first subset of cavities extending through intermediate layers        of the stack of layers, the second subset of cavities extending        entirely through the stack of layers; and    -   a vapor or a source of the vapor disposed in each of the first        subset of cavities;    -   wherein the stack of layers comprises:        -   a first end layer at a first end of the body, the first end            layer being formed of a first type of dielectric material            that is optically transparent;        -   a second end layer at a second, opposite end of the body,            the second end layer being formed of dielectric material;        -   the intermediate layers, positioned between the first and            second end layers and formed of dielectric material.-   Example 93. The vapor cell of example 92,    -   wherein the array of cavities reside at respective locations        defined by a two-dimensional lattice, the two dimensional        lattice comprising first and second lattice sites; and    -   wherein the first subset of cavities occupy respective first        lattice sites and the second subset of cavities occupy        respective second lattice sites.-   Example 94. The vapor cell of example 93, wherein all of the first    lattice sites are occupied by respective cavities of the first    subset and all of the second lattice sites are occupied by    respective cavities of the second subset.-   Example 95. The vapor cell of example 93 or example 94, wherein the    two dimensional lattice is a square lattice and the first and second    lattice sites alternate in sequence along rows and columns of the    square lattice.-   Example 96. The vapor cell of example 92, wherein the array of    cavities reside at respective locations of a square lattice and the    first and second subset of cavities alternate in sequence along rows    and columns of the square lattice.-   Example 97. The vapor cell of example 92 or any one of examples    93-95,    -   wherein each cavity of the first subset is encircled by a side        wall formed by the stack of layers, the side wall having four        exterior surfaces that define a square cross-section of the side        wall; and    -   wherein each cavity of the second subset has a square        cross-section.-   Example 98. The vapor cell of example 92 or any one of examples    93-97, wherein the second end layer is formed of the first type of    dielectric material.-   Example 99. The vapor cell of example 92 or any one of examples    93-98, wherein the intermediate layers comprise:    -   a first subset of layers formed of the first type of dielectric        material; and    -   a second subset of layers formed of a second, different type of        dielectric material.-   Example 100. The vapor cell of example 99, wherein the layers are    ordered in the stack to alternate between the first type of    dielectric material and the second type of dielectric material.-   Example 101. The vapor cell of example 99 or example 100, wherein    the second type of dielectric material is silicon.-   Example 102. The vapor cell of example 92 or any one of examples    93-101, wherein the first type of dielectric material comprises    silicon oxide.-   Example 103. The vapor cell of example 92 or any one of examples    93-102, wherein the stack of layers comprises:    -   a first set of tabs extending outward from the intermediate        layers at a first location of an exterior perimeter of the body;        and    -   a second set of tabs extending outward from the intermediate        layers at a second location of the exterior perimeter of the        body.-   Example 104. The vapor cell of example 103,    -   wherein each cavity of the first subset is encircled by a side        wall formed by the stack of layers, the side wall comprising a        corner that defines part of the exterior perimeter of the body;        and    -   wherein the first location is a first corner of the exterior        perimeter of the body and the second location is a second,        different corner of the exterior perimeter of the body.-   Example 105. The vapor cell of example 103,    -   wherein each cavity of the first subset is encircled by a side        wall formed by the stack of layers, the side wall comprising a        flat surface that defines part of the exterior perimeter of the        body; and    -   wherein the first location is a first flat surface of the        exterior perimeter of the body and the second location is a        second, different flat surface of the exterior perimeter of the        body.-   Example 106. The vapor cell of example 103 or any one of examples    104-105,    -   wherein the vapor cell is a first vapor cell;    -   wherein the first set of tabs defines a first mechanical        interface that is configured to interlock with a second vapor        cell (e.g., a second instance of the first vapor cell); and    -   wherein the second set of tabs defines a second mechanical        interface that is configured to interlock with a third vapor        cell (e.g., a third instance of the first vapor cell).-   Example 107. The vapor cell of example 103 or any one of examples    104-106,    -   wherein each tab of the first set is an integral part of the        intermediate layer from which it extends; and    -   wherein each tab of the second set is an integral part of the        intermediate layer from which it extends.-   Example 108. The vapor cell of example 103 or any one of examples    104-107,    -   wherein the intermediate layers comprise a first subset of        layers and a second subset of layers that alternate in sequence        along the stack of layers;    -   wherein each tab of the first set extends outward from a        respective layer in the first subset of layers; and    -   wherein each tab of the second set extends outward from a        respective layer in the second subset of layers.-   Example 109. A plurality of vapor cells, each corresponding to the    vapor cell of example 103 or any one of examples 104-108, in which    the first and second sets of tabs of each vapor cell are interlocked    with, respectively, the second set of tabs of a first vapor cell and    the first set of tabs of a second, different vapor cell.-   Example 110. The vapor cell of example 92 or any one of examples    93-109,    -   wherein each cavity of the first subset has a first opening        adjacent the first end layer;    -   wherein the first end layer comprises:        -   an interior surface covering the first openings of the first            subset of cavities, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 111. The vapor cell of example 110, wherein the interior    surface of the first end layer comprises a reflective coating    disposed thereon.-   Example 112. The vapor cell of example 110 or example 111, wherein    the exterior surface of the first end layer comprises a reflective    coating disposed thereon.-   Example 113. The vapor cell of example 110 or any one of examples    111-112, wherein the interior surface of the first end layer    comprises an anti-reflective coating disposed thereon.-   Example 114. The vapor cell of example 110 or any one of examples    111-113, wherein the exterior surface of the first end layer    comprises an anti-reflective coating disposed thereon.-   Example 115. The vapor cell of example 92 or any one of examples    93-114,    -   wherein each cavity of the first subset has a second opening        adjacent the second end layer;    -   wherein the second end layer comprises:        -   an interior surface covering the second openings of the            first subset of cavities, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 116. The vapor cell of example 115, wherein the interior    surface of the second end layer comprises a reflective coating    disposed thereon.-   Example 117. The vapor cell of example 115 or example 116, wherein    the exterior surface of the second end layer comprises a reflective    coating disposed thereon.-   Example 118. The vapor cell of example 115 or any one of examples    116-117, wherein the interior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 119. The vapor cell of example 115 or any one of examples    116-118, wherein the exterior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 120. The vapor cell of example 92 or any one of examples    93-119,    -   wherein each cavity of the first subset is encircled by a side        wall formed by the stack of layers, the side wall having one or        more exterior surfaces that define at least part of an exterior        unit-cell perimeter that encircles the cavity; and    -   wherein, for one or more cavities of the first subset, at least        one intermediate layer comprises a plurality of holes between        the one or more cavities and their respective exterior unit-cell        perimeters.-   Example 121. The vapor cell of example 120, wherein the plurality of    holes extends through the at least one intermediate layer.-   Example 122. The vapor cell of example 120 or example 121,    -   wherein the vapor cell is configured to detect a target        radiation; and    -   wherein each of the plurality of holes has a largest dimension        no greater than a wavelength of the target radiation.-   Example 123. The vapor cell of example 122, wherein the target    radiation has a wavelength of at least 0.3 mm.-   Example 124. The vapor cell of example 92 or any one of examples    93-123, wherein an interface between at least one pair of adjacent    layers in the stack comprises an adhesion layer.-   Example 125. The vapor cell of example 124, wherein the adhesion    layer comprises silicon oxide.-   Example 126. The vapor cell of example 92 or any one of examples    93-125,    -   wherein each intermediate layer comprises a through-hole that        defines a portion of a cavity of the first subset through the        intermediate layer; and    -   wherein at least two adjacent intermediate layers have        respective through-holes for the cavity of the first subset that        differ, relative to each other, in one or both of shape and        size.-   Example 127. The vapor cell of example 92 or any one of examples    93-126,    -   wherein each intermediate layer comprises a through-hole that        defines a portion of a cavity of the second subset through the        intermediate layer; and    -   wherein at least two adjacent intermediate layers have        respective through-holes for the cavity of the second subset        that differ, relative to each other, in one or both of shape and        size.

In some aspects of what is described, a method of manufacturing a vaporcell may be described by the following examples:

-   Example 128. A method of manufacturing a vapor cell, the method    comprising:    -   obtaining a stack of layers bonded to each other and defining an        array of cavities that comprises a first subset of cavities and        a second subset of cavities, the first subset of cavities        extending through intermediate layers of stack of layers, the        second subset of cavities extending entirely through the stack        of layers, the stack of layers comprising:        -   a first end layer formed of a first type of dielectric            material that is optically transparent, and        -   the intermediate layers, formed of dielectric material and            extending in sequence from a first intermediate layer to a            last intermediate layer, wherein:            -   the first intermediate layer is bonded to the first end                layer, and            -   each of the first subset of cavities comprises an                opening defined by a surface of the last intermediate                layer;    -   disposing a vapor or a source of the vapor into each of the        first subset of cavities; and    -   bonding a second end layer of the stack of layers to the last        intermediate layer to seal the vapor or the source of the vapor        in the first subset of cavities, the second end layer formed of        dielectric material;    -   wherein each cavity of the second subset extends through the        second end layer; and    -   wherein the stack of layers, when comprising the bonded second        end layer, defines a body of the vapor cell, the first end layer        at a first end of the body and the second end layer at a second,        opposite end of the body.-   Example 129. The method of example 128, wherein bonding the second    end layer comprises covering each opening of the first subset of    cavities with the second end layer to enclose the vapor or the    source of the vapor in the first subset of cavities.-   Example 130. The method of example 128 or example 129, wherein the    second end layer is formed of the first type of dielectric material.-   Example 131. The method of example 128 or any one of examples    129-130, wherein the first type of dielectric material comprises    silicon oxide.-   Example 132. The method of example 128 or any one of examples    129-131, wherein obtaining the stack of layers comprises:    -   removing material from a first wafer formed of the first type of        dielectric material to fabricate the first end layer, the second        end layer (when combined with Example 130), or both; and    -   removing material from a second wafer formed of dielectric        material to fabricate the intermediate layers.-   Example 133. The method of example 128 or any one of examples    129-131, wherein the intermediate layers comprise:    -   a first subset of layers formed of the first type of dielectric        material; and    -   a second subset of layers formed of a second, different type of        dielectric material.-   Example 134. The method of example 133, wherein obtaining the stack    of layers comprises:    -   alternately bonding layers of the first and second subset of        layers to each other to form the intermediate layers; and    -   bonding the first intermediate layer to the first end layer.-   Example 135. The method of example 133 or example 134, wherein    obtaining the stack of layers comprises:    -   removing material from a first wafer formed of the first type of        dielectric material to fabricate the first end layer, one or        more of the first subset of layers, the second end layer (when        combined with Example 130), or any combination thereof; and    -   removing material from a second wafer formed of the second type        of dielectric material to fabricate one or more of the second        subset of layers.-   Example 136. The method of example 133 or any one of examples    134-135, wherein the second type of dielectric material is silicon.-   Example 137. The method of example 128 or any one of examples    129-136, wherein bonding the second end layer comprises:    -   altering the surface of the last intermediate layer to comprise        a first plurality of hydroxyl ligands;    -   altering a surface of the second end layer to comprise a second        plurality of hydroxyl ligands; and    -   contacting the altered surfaces to form a seal around each        opening of the first subset of cavities, the seal defined by        metal-oxygen bonds formed by reacting the first plurality of        hydroxyl ligands with the second plurality of hydroxyl ligands        during contact of the two altered surfaces.-   Example 138. The method of example 137, wherein bonding the second    end layer comprises:    -   activating one or both of the surfaces of the last intermediate        layer and the second end layer by exposing their respective        surfaces to a plasma.-   Example 139. The method of example 128 or any one of examples    129-138,    -   wherein each cavity of the first subset is encircled by a side        wall formed by the stack of layers, the side wall having one or        more exterior surfaces that define at least part of an exterior        unit-cell perimeter that encircles the cavity; and    -   wherein, for one or more cavities of the first subset, at least        one intermediate layer comprises a plurality of holes between        the one or more cavities and their respective exterior unit-cell        perimeters.-   Example 140. The method of example 139, wherein the plurality of    holes extends through the at least one intermediate layer.-   Example 141. The method of example 139 or example 140,    -   wherein the vapor cell, when manufactured, is configured to        detect a target radiation; and    -   wherein each of the plurality of holes has a largest dimension        no greater than a wavelength of the target radiation.-   Example 142. The method of example 141, wherein the target radiation    has a wavelength of at least 0.3 mm.-   Example 143. The method of example 128 or any one of examples    129-142, wherein obtaining the stack of layers comprises:    -   forming an adhesion layer on one or both sides of an        intermediate layer to define respective bonding surfaces of the        intermediate layer.-   Example 144. The method of example 143, wherein the adhesion layer    comprises silicon oxide.-   Example 145. The method of example 128 or any one of examples    129-144, wherein the stack of layers comprises:    -   a first set of tabs extending outward from the intermediate        layers at a first location of an exterior perimeter of the stack        of layers (or body of the vapor cell); and    -   a second set of tabs extending outward from the intermediate        layers at a second location of the exterior perimeter of the        stack of layers (or body of the vapor cell).-   Example 146. The method of example 145,    -   wherein the vapor cell is a first vapor cell;    -   wherein the first set of tabs defines a first mechanical        interface that is configured to interlock with a second vapor        cell (e.g., a second instance of the first vapor cell); and    -   wherein the second set of tabs defines a second mechanical        interface that is configured to interlock with a third vapor        cell (e.g., a third instance of the first vapor cell).

In some aspects of what is described, a vapor cell may be described bythe following examples:

-   Example 147. A vapor cell, comprising:    -   a body defined by a stack of layers bonded to each other, the        stack of layers comprising:        -   a first end layer at a first end of the body, the first end            layer being formed of a first type of dielectric material            that is optically transparent,        -   a second end layer at a second, opposite end of the body,            the second end layer being formed of dielectric material,        -   intermediate layers between the first and second end layers            and formed of dielectric material, wherein:            -   the intermediate layers define an internal cavity that                extends through the body between the first end layer and                the second end layer, and            -   each intermediate layer comprises a through-hole that                defines a portion of the internal cavity through the                intermediate layer; and    -   a vapor or a source of the vapor disposed in the internal        cavity.-   Example 148. The vapor cell of example 147, wherein each    through-hole is identical in shape and size.-   Example 149. The vapor cell of example 147, wherein at least two    adjacent intermediate layers have respective through-holes that    differ, relative to each other, in one or both of shape and size.-   Example 150. The vapor cell of example 147,    -   wherein the through-holes are circular and are aligned along a        direction perpendicular to the stack of layers; and    -   wherein the through-holes vary in size to define a spherical        volume for the internal cavity.-   Example 151. The vapor cell of example 147 or any one of examples    148-150, wherein the second end layer is formed of the first type of    dielectric material.-   Example 152. The vapor cell of example 147 or any one of examples    148-151, wherein the intermediate layers comprise:    -   a first subset of layers formed of the first type of dielectric        material; and    -   a second subset of layers formed of a second, different type of        dielectric material.-   Example 153. The vapor cell of example 152, wherein the layers are    ordered in the stack to alternate between the first type of    dielectric material and the second type of dielectric material.-   Example 154. The vapor cell of example 152 or example 153, wherein    the second type of dielectric material is silicon.-   Example 155. The vapor cell of example 147 or any one of examples    148-154, wherein the first type of dielectric material comprises    silicon oxide.-   Example 156. The vapor cell of example 147 or any one of examples    148-155, wherein the stack of layers comprises:    -   a first set of tabs extending outward from the intermediate        layers on a first exterior side (or location) of the body; and    -   a second set of tabs extending outward from the intermediate        layers on a second exterior side (or location) of the body.-   Example 157. The vapor cell of example 156,    -   wherein the vapor cell is a first vapor cell;    -   wherein the first set of tabs defines a first mechanical        interface that is configured to interlock with a second vapor        cell (e.g., a second instance of the first vapor cell); and    -   wherein the second set of tabs defines a second mechanical        interface that is configured to interlock with a third vapor        cell (e.g., a third instance of the first vapor cell).-   Example 158. The vapor cell of example 156 or example 157,    -   wherein each tab of the first set is an integral part of the        intermediate layer from which it extends; and    -   wherein each tab of the second set is an integral part of the        intermediate layer from which it extends.-   Example 159. The vapor cell of example 156 or any one of examples    157-158, wherein the first and second sets of tabs are aligned with    respective centers of the first and second exterior sides.-   Example 160. The vapor cell of example 156 or any one of examples    157-159,    -   wherein the intermediate layers comprise a first subset of        layers and a second subset of layers that alternate in sequence        along the stack of layers;    -   wherein each tab of the first set extends outward from a        respective layer in the first subset of layers; and    -   wherein each tab of the second set extends outward from a        respective layer in the second subset of layers.-   Example 161. A plurality of vapor cells, each corresponding to the    vapor cell of example 156 or any one of examples 157-160, in which    the first and second sets of tabs of each vapor cell are interlocked    with, respectively, the second set of tabs of a first vapor cell and    the first set of tabs of a second, different vapor cell.-   Example 162. The vapor cell of example 147 or any one of examples    148-161,    -   wherein the body has four exterior sides defining a square        cross-section of the body, the four exterior sides comprising        first and second exterior sides; and    -   wherein the stack of layers comprises:        -   a first set of tabs extending outward from the intermediate            layers on the first exterior side of the body, and        -   a second set of tabs extending outward from the intermediate            layers on the second exterior side of the body.-   Example 163. The vapor cell of example 162, wherein the first and    second exterior sides are adjacent to each other.-   Example 164. The vapor cell of example 162, wherein the first and    second exterior sides are opposite each other.-   Example 165. The vapor cell of example 162 or any one of examples    163-164,    -   wherein the four exterior sides comprise third and fourth        exterior sides; and    -   wherein the stack of layers comprises:        -   a third set of tabs extending outward from the intermediate            layers on the third exterior side of the body, and        -   a fourth set of tabs extending outward from the intermediate            layers on the fourth exterior side of the body.-   Example 166. The vapor cell of example 165,    -   wherein each tab of the third set is an integral part of the        intermediate layer from which it extends; and    -   wherein each tab of the fourth set is an integral part of the        intermediate layer from which it extends.-   Example 167. The vapor cell of example 165 or example 166, wherein    the first, second, third, and fourth sets of tabs are aligned with    respective centers of the first, second, third, and fourth exterior    sides.-   Example 168. The vapor cell of example 165 or any one of examples    166-167,    -   wherein the intermediate layers comprise a first subset of        layers and a second subset of layers that alternate in sequence        along the stack of layers;    -   wherein each tab of the first and third sets extends outward        from a respective layer in the first subset of layers; and    -   wherein each tab of the second and fourth sets extends outward        from a respective layer in the second subset of layers.-   Example 169. The vapor cell of example 147 or any one of examples    148-168,    -   wherein the first end layer comprises:        -   an interior surface covering a first opening of the internal            cavity adjacent the first end layer, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 170. The vapor cell of example 169, wherein the interior    surface of the first end layer comprises a reflective coating    disposed thereon.-   Example 171. The vapor cell of example 169 or example 170, wherein    the exterior surface of the first end layer comprises a reflective    coating disposed thereon.-   Example 172. The vapor cell of example 169 or any one of examples    170-171, wherein the interior surface of the first end layer    comprises an anti-reflective coating disposed thereon.-   Example 173. The vapor cell of example 169 or any one of examples    170-172, wherein the exterior surface of the first end layer    comprises an anti-reflective coating disposed thereon.-   Example 174. The vapor cell of example 147 or any one of examples    148-173,    -   wherein the second end layer comprises:        -   an interior surface covering a second opening of the            internal cavity adjacent the second end layer, and        -   an exterior surface opposite the interior surface; and    -   wherein one or both of the interior and exterior surfaces have        an optical coating disposed thereon.-   Example 175. The vapor cell of example 174, wherein the interior    surface of the second end layer comprises a reflective coating    disposed thereon.-   Example 176. The vapor cell of example 174 or example 175, wherein    the exterior surface of the second end layer comprises a reflective    coating disposed thereon.-   Example 177. The vapor cell of example 174 or any one of examples    175-176, wherein the interior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 178. The vapor cell of example 174 or any one of examples    175-177, wherein the exterior surface of the second end layer    comprises an anti-reflective coating disposed thereon.-   Example 179. The vapor cell of example 147 or any one of examples    148-178, wherein at least one of the intermediate layers comprises a    plurality of holes between the internal cavity and an outer    perimeter of the intermediate layer.-   Example 180. The vapor cell of example 179, wherein the plurality of    holes extends through the intermediate layer.-   Example 181. The vapor cell of example 179 or example 180,    -   wherein the vapor cell is configured to detect a target        radiation; and    -   wherein each of the plurality of holes has a largest dimension        no greater than a wavelength of the target radiation.-   Example 182. The vapor cell of example 181, wherein the target    radiation has a wavelength of at least 0.3 mm.-   Example 183. The vapor cell of example 147 or any one of examples    148-182, wherein an interface between at least one pair of adjacent    layers in the stack comprises an adhesion layer.-   Example 184. The vapor cell of example 183, wherein the adhesion    layer comprises silicon oxide.-   Example 185. The vapor cell of example 147 or any one of examples    148-184,    -   wherein the body comprises an outer shape; and    -   wherein each layer in the stack comprises an outer perimeter        surface that defines a cross-section of an outer shape at a        location of the layer.-   Example 186. The vapor cell of example 185, wherein the    cross-section of the outer shape is constant along the stack of    layers.-   Example 187. The vapor cell of example 185, wherein at least two    adjacent intermediate layers have respective outer perimeter    surfaces that differ, relative to each other, in one or both of    shape and size.-   Example 188. The vapor cell of example 185 (but excluding the    features of Examples 162-168), wherein the cross-section is circular    in shape and varies in size along the stack of layers to define an    outer spherical shape for the body.

In some aspects of what is described, a method of manufacturing a vaporcell may be described by the following examples:

-   Example 189. A method of manufacturing a vapor cell, the method    comprising:    -   obtaining a stack of layers bonded to each other and comprising:        -   a first end layer formed of a first type of dielectric            material that is optically transparent,        -   intermediate layers formed of dielectric material and            extending in sequence from a first intermediate layer to a            last intermediate layer, wherein:            -   the first intermediate layer is bonded to the first end                layer,            -   an internal cavity extends through the intermediate                layers and comprises an opening defined by a surface of                the last intermediate layer, and            -   each intermediate layer comprises a through-hole that                defines a portion of the internal cavity through the                intermediate layer; and    -   disposing a vapor or a source of the vapor into the internal        cavity; and    -   bonding a second end layer of the stack of layers to the last        intermediate layer to seal the vapor or the source of the vapor        in the internal cavity, the second end layer formed of        dielectric material;    -   wherein the stack of layers, when comprising the bonded second        end layer, defines a body of the vapor cell, the first end layer        at a first end of the body and the second end layer at a second,        opposite end of the body.-   Example 190. The method of example 189, wherein each through-hole is    identical in shape and size.-   Example 191. The method of example 189, wherein at least two    adjacent intermediate layers have respective through-holes that    differ, relative to each other, in one or both of shape and size.-   Example 192. The method of example 191,    -   wherein the through-holes are circular and are aligned along a        direction perpendicular to the stack of layers; and    -   wherein the through-holes vary in size to define a spherical        volume for the internal cavity.-   Example 193. The method of example 189 or any one of examples    190-192, wherein bonding the second end layer comprises covering the    opening of the internal cavity with the second end layer to enclose    the vapor or the source of the vapor in the internal cavity.-   Example 194. The method of example 189 or any one of examples    190-193, wherein the second end layer is formed of the first type of    dielectric material.-   Example 195. The method of example 189 or any one of examples    190-194, wherein the first type of dielectric material comprises    silicon oxide.-   Example 196. The method of example 189 or any one of examples    190-195, wherein obtaining the stack of layers comprises:    -   removing material from a first wafer formed of the first type of        dielectric material to fabricate the first end layer, the second        end layer (when combined with Example 194), or both; and    -   removing material from a second wafer formed of dielectric        material to fabricate the intermediate layers.-   Example 197. The method of example 189 or any one of examples    190-195, wherein the intermediate layers comprise:    -   a first subset of layers formed of the first type of dielectric        material; and    -   a second subset of layers formed of a second, different type of        dielectric material.-   Example 198. The method of example 197, wherein obtaining the stack    of layers comprises:    -   alternately bonding layers of the first and second subset of        layers to each other to form the intermediate layers; and    -   bonding the first intermediate layer to the first end layer.-   Example 199. The method of example 197 or example 198, wherein    obtaining the stack of layers comprises:    -   removing material from a first wafer formed of the first type of        dielectric material to fabricate the first end layer, one or        more of the first subset of layers, the second end layer (when        combined with Example 194), or any combination thereof; and    -   removing material from a second wafer formed of the second type        of dielectric material to fabricate one or more of the second        subset of layers.-   Example 200. The method of example 197 or any one of examples    198-199, wherein the second type of dielectric material is silicon.-   Example 201. The method of example 189 or any one of examples    190-200, wherein bonding the second end layer comprises:    -   altering the surface of the last intermediate layer to comprise        a first plurality of hydroxyl ligands;    -   altering a surface of the second end layer to comprise a second        plurality of hydroxyl ligands; and    -   contacting the altered surfaces to form a seal around the        opening of the internal cavity, the seal defined by metal-oxygen        bonds formed by reacting the first plurality of hydroxyl ligands        with the second plurality of hydroxyl ligands during contact of        the two altered surfaces.-   Example 202. The method of example 201, wherein bonding the second    end layer comprises:    -   activating one or both of the surfaces of the last intermediate        layer and the second end layer by exposing their respective        surfaces to a plasma.-   Example 203. The method of example 189 or any one of examples    190-202, wherein at least one of the intermediate layers comprises a    plurality of the holes between the internal cavity and an outer    perimeter of the intermediate layer.-   Example 204. The method of example 203, wherein the plurality of    holes extends through the intermediate layer.-   Example 205. The method of example 203 or example 204,    -   wherein the vapor cell, when manufactured, is configured to        detect a target radiation; and    -   wherein each of the plurality of holes has a largest dimension        no greater than a wavelength of the target radiation.-   Example 206. The method of example 205, wherein the target radiation    has a wavelength of at least 0.3 mm.-   Example 207. The method of example 189 or any one of examples    190-206, wherein obtaining the stack of layers comprises:    -   forming an adhesion layer on one or both sides of an        intermediate layer to define respective bonding surfaces of the        intermediate layer.-   Example 208. The method of example 207, wherein the adhesion layer    comprises silicon oxide.-   Example 209. The method of example 189 or any one of examples    190-208, wherein the stack of layers comprises:    -   a first set of tabs extending outward from the intermediate        layers on a first exterior side of the stack of layers (or body        of the vapor cell); and    -   a second set of tabs extending outward from the intermediate        layers on a second exterior side of the stack of layers (or body        of the vapor cell).-   Example 210. The method of example 209,    -   wherein the vapor cell is a first vapor cell;    -   wherein the first set of tabs defines a first mechanical        interface that is configured to interlock with a second vapor        cell (e.g., a second instance of the first vapor cell); and    -   wherein the second set of tabs defines a second mechanical        interface that is configured to interlock with a third vapor        cell (e.g., a third instance of the first vapor cell).-   Example 211. The method of example 189 or any one of examples    190-210,    -   wherein the body comprises an outer shape; and    -   wherein each layer in the stack comprises an outer perimeter        surface that defines a cross-section of an outer shape at a        location of the layer.-   Example 212. The method of example 211, wherein the cross-section of    the outer shape is constant along the stack of layers.-   Example 213. The method of example 211, wherein at least two    adjacent intermediate layers have respective outer perimeter    surfaces that differ, relative to each other, in one or both of    shape and size.-   Example 214. The method of example 211, wherein the cross-section is    circular in shape and varies in size along the stack of layers to    define an outer spherical shape for the body.

While this specification contains many details, these should not beunderstood as limitations on the scope of what may be claimed, butrather as descriptions of features specific to particular examples.Certain features that are described in this specification or shown inthe drawings in the context of separate implementations can also becombined. Conversely, various features that are described or shown inthe context of a single implementation can also be implemented inmultiple embodiments separately or in any suitable sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single product or packagedinto multiple products.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications can be made. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A method of manufacturing a vapor cell, themethod comprising: obtaining a stack of layers bonded to each other andcomprising: a first end layer formed of a first type of dielectricmaterial that is optically transparent, intermediate layers formed ofdielectric material and extending in sequence from a first intermediatelayer to a last intermediate layer, wherein: the first intermediatelayer is bonded to the first end layer, and an internal cavity extendsthrough the intermediate layers and comprises an opening defined by asurface of the last intermediate layer, a first set of tabs extendingoutward from the intermediate layers on a first exterior side of thestack of layers, and a second set of tabs extending outward from theintermediate layers on a second exterior side of the stack of layers;disposing a vapor or a source of the vapor into the internal cavity; andbonding a second end layer of the stack of layers to the lastintermediate layer to seal the vapor or the source of the vapor in theinternal cavity, the second end layer formed of dielectric material;wherein the stack of layers, when comprising the bonded second endlayer, defines a body of the vapor cell, the first end layer at a firstend of the body and the second end layer at a second, opposite end ofthe body.
 2. The method of claim 1, wherein bonding the second end layercomprises covering the opening of the internal cavity with the secondend layer to enclose the vapor or the source of the vapor in theinternal cavity.
 3. The method of claim 1, wherein the second end layeris formed of the first type of dielectric material.
 4. The method ofclaim 1, wherein the intermediate layers comprise: a first subset oflayers formed of the first type of dielectric material; and a secondsubset of layers formed of a second, different type of dielectricmaterial.
 5. The method of claim 4, wherein obtaining the stack oflayers comprises: alternately bonding layers of the first and secondsubset of layers to each other to form the intermediate layers; andbonding the first intermediate layer to the first end layer.
 6. Themethod of claim 4, wherein obtaining the stack of layers comprises:removing material from a first wafer formed of the first type ofdielectric material to fabricate the first end layer, one or more of thefirst subset of layers, or both; and removing material from a secondwafer formed of the second type of dielectric material to fabricate oneor more of the second subset of layers.
 7. The method of claim 1,wherein bonding the second end layer comprises: altering the surface ofthe last intermediate layer to comprise a first plurality of hydroxylligands; altering a surface of the second end layer to comprise a secondplurality of hydroxyl ligands; and contacting the altered surfaces toform a seal around the opening of the internal cavity, the seal definedby metal-oxygen bonds formed by reacting the first plurality of hydroxylligands with the second plurality of hydroxyl ligands during contact ofthe two altered surfaces.
 8. The method of claim 7, wherein bonding thesecond end layer comprises: activating one or both of the surfaces ofthe last intermediate layer and the second end layer by exposing theirrespective surfaces to a plasma.
 9. The method of claim 1, wherein atleast one of the intermediate layers holes comprises a plurality ofholes between the internal cavity and an outer perimeter of theintermediate layer.
 10. The method of claim 9, wherein the plurality ofholes extend through the intermediate layer.
 11. The method of claim 1,wherein obtaining the stack of layers comprises: forming an adhesionlayer on one or both sides of an intermediate layer to define respectivebonding surfaces of the intermediate layer.